At the heart of the philosophy of St. Thomas Aquinas lies his natural theology. The linchpin of that theology is his Five Ways--his arguments for the existence of God. The presupposition behind all this is the belief that God's existence can be demonstrated on theologically neutral grounds. If God's existence cannot be demonstrated, then neither can His attributes. It is no accident that Aquinas' arguments for God's existence precede his discussion of God's attributes.  Without the former, his demonstration of the latter would be impossible.
Many twentieth century evangelicals have been attracted to the natural theology of Aquinas, agreeing with him at least in their belief that God's existence can be demonstrated on theologically neutral grounds. The list includes Stuart C. Hackett,  Norman Geisler,  R.C. Sproul,  J.P. Moreland, [5 ] and William Lane Craig.  In light of the resurgence of the Thomistic arguments and their attractiveness for evangelicals, we must examine afresh its approach to the apologetic task.
Those familiar with the history of apologetics of the past couple centuries are certainly aware that this procedure flies directly in the face of much contemporary thought. Subsequent to Hume's and Kant's seemingly devastating critique of the various theistic "proofs," Christian apologists were generally content to assume that a theistic world view could not be established through the use of reason but could only be accepted on faith. Whatever their differences, men such as Soren Kierkegaard, William James, and Karl Barth agreed on one crucial point--theism could not be rationally demonstrated. Because of this shared belief, their approach is sometimes called fideism. Broadly speaking, a fideist in apologetics is one who believes that because the truth of theism cannot be demonstrated, it must be accepted by a "leap of faith" (to use Kierkegaard's phrase). 
What, then, of the attempts to resuscitate the traditional proofs? Can a viable apologetic proceed along these lines? Can a foundation be established which will provide the groundwork for a Thomistic natural theology?
Before looking at the specific arguments for God's existence proposed by Aquinas and his present-day evangelical followers, I would like to examine an assumption which I believe underlies all rationalistic apologetics. (I am using rationalistic broadly here to encompass all apologetics which assume that the truth of theism can be demonstrated on theologically neutral grounds) This assumption may be stated as follows:
(P) There exists a set of theological propositions, S, which can be rationally demonstrated.
Some comments are in order.
First, apologists may differ on the extent of S. All rationalistic apologists agree that S includes the proposition "God exists." Most believe S is considerably broader. Aquinas, for example, believed God's existence and His attributes could be demonstrated. Others have gone further still. Bonaventure thought "God is Triune" and "Jesus Christ is God" were to be included in S. So, apparently, does Geisler.  Craig appears to hold to the demonstrability of the bodily resurrection of Jesus.  If the minimal proposition "God exists" is not included in S, it would seem to follow that S is a null set.
Second, "can be rationally demonstrated" means "logically demonstrable to all rational people in ideal circumstances." Let's unpack this a bit.
(a) A proposition in S is "logically demonstrable" if it can be validly deduced from true premises. (Logicians refer to an argument with true premises and a valid inference as a sound argument.)
(b) "Rational people" are those who are capable of recognizing certain premises as true and others as false and can make valid inferences.
(c) Nobody is completely rational in the sense just defined. A person may not have access to all the data necessary to determine the truth or falsity of a premise and may therefore suppose a false premise is true or vice versa. Or, he may not be able to determine whether or not an inference is valid. (This is particularly true of extremely complex arguments or of those couched in a symbolic notation unfamiliar to the one to whom the argument has been presented.) This is why the caveat "ideal circumstances" is needed. An otherwise rational person may not be persuaded by a sound argument if he is ignorant of certain relevant data or doesn't have the capacity (for whatever reason) to make the inference. Such a person is not in "ideal circumstances" in relation to the argument.
The rationalistic apologist, then, is not committed to saying that all rational people will accept S, but only that those who are in ideal circumstances will.
Of course, many rational people do not accept the most minimal S-proposition --"God exists." Of those who do, a significant number believe that the rationalist's purported demonstrations are logically unsound. If (P) were true, this would mean that such people either did not possess the relevant data or were unable to draw a valid inference.
However, the typical rationalistic proofs for the existence of God have straightfoward inferences. They bear much more resemblance to
All men are mortal.
Socrates is a man.
Therefore, Socrates is mortal
than to a complex syllogism of math logic. Moreover, their premises generally are easily verifiable. Rational people, then, are typically in ideal circumstances vis-a-vis the rationalistic proofs. Some reason other than lack of perspicuity must be given for why these alleged proofs are unpersuasive.
The evangelicals upon whom we are focusing by and large reject the so-called ontological argument for God's existence.  Following Aquinas (who also rejects this argument), they have espoused one form or another of what is commonly known as the cosmological argument.
Although Aquinas presents Five Ways to prove God's existence, the first three have received the most attention and have had the greatest influence on contemporary evangelicals. We will accordingly focus on them as well.
Each of these Ways begins with a readily observable feature (or features) of the world and tries to demonstrate therefrom the existence of God.
The First Way is sometimes called the argument from motion or change. The feature of the world with which it begins is that every finite being changes. When something changes, however, it is going from its potential state to its actual state. It cannot do this on its own. This actualization must be caused by something else that has already passed from its state of potentiality to actuality. This second being must itself have been in a state of potentiality at one time and have passed to its state of actuality through the agency of yet another being.
Although Thomas uses confusing terminology (viz., "potentiality" and "actuality"), his point is really quite simple. Finite beings are always changing. The change in any given being, A, cannot be self-caused. We must assume that another being, B, caused this change. However, to cause change in A, B must also have undergone change. This can only be explained through positing another being C. And so forth.
Thomas proceeds to argue as follows:
Now we must stop somewhere, otherwise there will be no first cause of the change, and, as a result, no subsequent cause. For it is only when acted upon by the first cause that the intermediate causes will produce the change...Hence one is bound to arrive at some first cause of change not itself being changed by anything, and this is what everybody understands by God. 
A word of clarification is in order. Aquinas is not arguing for a first temporal cause. (As a matter of fact, he does not believe that the non-eternity of the world can be demonstrated apart from revelation.) Rather, he is arguing for a first sustaining cause. All finite things are dependent for their change on something else. So long as we explain this change by appeal to an "intermediate cause," we will not have given a full explanation. Ultimately, something that can sustain change in finite objects but is not itself changing must be posited. This first sustaining cause is God.
The argument of the Second Way is similar to the First. The major difference is that while the First Way talks about the cause of change or motion, the Second focuses on the cause of the existence of things. Finite beings are not self-existent. They are caused to exist by something else. However, so long as we appeal to other finite beings, we have not given an adequate explanation. To fully explain the existence of any and every finite being, a self-existent being must be posited. This self-existent being is God. Again, the idea of cause at work here is not temporal cause but sustaining cause. A first sustaining cause is necessary to explain the ongoing existence of finite beings.
The Third Way shows that the being whose existence is being established is a necessary being. The argument here proceeds by pointing out the fact that this world is populated by contingent beings--i.e., beings which are but need not be. But, as Aquinas notes:
A thing that need not be, once was not; and if everything need not be, once upon a time there was nothing. But if this were true there would be nothing even now, because something that does not exist can only be brought into being by something already existing. So that if nothing was in being nothing could be brought into being, and nothing would be in being now, which contradicts observation. Not everything therefore is the sort of thing that need not be; there has got to be something that must be. 
Something that must be is, of course, a necessary being.
Aquinas' argument in the Third Way may be clarified as follows:
Either there is a necessary being or there isn't. Suppose there is no necessary being. There is either a first contingent being or there is not. If there is a first contingent being and no necessary being, the first contingent being would have had to pop out of nothing. This is impossible. Ex nihilo nihil fit. Therefore, if at one time nothing existed, nothing would exist now.
If there is no first contingent being (perhaps because the contingent beings comprise an interrelated whole) and no necessary being, each contingent being will depend for its existence on another contingent being but not on any necessary being. But, not even an interrelated sum of contingent beings can provide a fully adequate explanation for the existence of other contingent beings. The existence of that interrelated whole (and therefore of each one of them) still must be explained. Only if a non-contingent, i.e., a necessary being exists can this explanation be given.
Before evaluating the cosmological argument, I would like to schematize it. I have tried to account for what Aquinas is saying in his first Three Ways, but I am also drawing on contemporary formulations.  In so doing, I am trying to present the argument in its strongest possible form. This argument, which we'll call Argument A, runs as follows:
(1) Contingent, non-self-existent, changing beings exist.
(2) Their existence implies the existence of other beings upon whom they depend for their existence and their change.
(3) The sum of contingent, non-self-existent, changing beings upon which any given being depends either has a first member or it does not.
(4) If it has a first member, a necessary, self-existent, unchanging being must exist. Otherwise, the first member could not exist since the first member could not pop out of nothing. Ex nihilo nihil fit.
(5) If it does not have a first member, a necessary, self-existent unchanging being must exist. Otherwise, the contingency, non-self-existence, and changing nature of the interrelated whole would not be explained. ("If the parts are contingent, then the whole is contingent."  )
Therefore, from (3)-(5) it follows that a necessary, self-existent, unchanging being must exist. This being is God.
Those criticisms which challenge the supposed impossibility of an infinite temporal series of causes  are irrelevant to this formulation. At issue here is not temporal but sustaining causation.
Another unpersuasive criticism is the one which focuses on the concept of a necessary being. It alleges that the cosmological argument fails because a necessary being is a contradiction in terms. Propositions can be necessary; beings can't.
This criticism confuses two types of necessity--logical and ontological necessity. We may grant that propositions alone can be logically necessary. This says nothing about whether beings can possess a different sort of necessity. An ontologically necessary being is one whose existence is necessary for the existence of contingent beings. By definition, it possesses properties opposite those which make a being contingent--e.g., it is uncaused, independent, indestructible, incorruptible. etc. To claim that no such being could exist is simply to assert the contrary to the conclusion of the cosmological argument without defending this assertion. Such a procedure is question-begging. 
Some critics have argued that positing a necessary being to explain finite beings doesn't solve anything. Its existence too must be explained by appeal to yet another being. And so on, ad infinitum. Since the quest for explanatory adequacy is bound to fail, we may as well stop with the world of finite beings.
This criticism also fails. By definition, a necessary being is uncaused, independent, and so forth. It is the one being that can account for the existence of other beings but does not itself need to be accounted for. Therefore, positing it does not start us on the road to an infinite regress but rather forestalls that very regress.
Yet another criticism is that even if the cosmological argument were sound, natural theologians, notoriously, do not demonstrate the existence of the God of Scripture but some impersonal idol. The cosmological argument is thus unbiblical and as Van Til has stated, "We are frankly out to establish the truth of Christian theism not theism in general. We are Protestants." 
Nevertheless, the cosmological argument ought to be rejected on other grounds as well. The above schematization of the cosmological argument makes two assumptions which, although initially persuasive, are not logically necessary. The first is ex nihilo nihil fit. The second is, "If the parts are contingent, then the whole is contingent." Let's look at each of these in turn.
1. Ex nihilo nihil fit. The cosmological argument makes this assumption about a first contingent being. The following argument (Argument B) shows the contrary:
(1') Contingent beings exist.
(2') Their existence implies the existence of other beings upon which they depend for their existence.
(3') Because no necessary being exists at least one otherwise contingent being does not depend for its existence on another being.
(4') Because this being is not necessary, its existence must be explained somehow.
(5') The only explanation is that it simply comes into existence.
Therefore, it is false that ex nihilo nihil fit.
This argument, of course, assumes the truth of atheism and is therefore question-begging. However, if we examine Argument A, we find that its premise (4) is equally question-begging. Let's note that premise again:
(4) If the sum of contingent beings has a first member, a necessary, self-existent, unchanging being must exist. Otherwise, the first member could not exist since the first member could not pop out of nothing. Ex nihilo nihil fit.
An atheistic universe with a first member requires that at least this first member simply come into existence, as Argument B demonstrates. However, to assume that nothing can come from nothing, as premise (4) of Argument A does, is to guarantee the theistic conclusion in advance. By definition, the first member cannot be caused by another finite being. If it were, it would not be the first member of the chain. However, to assume that the first member must be caused (which is what ex nihilo nihil fit amounts to) is to assume that a necessary being must exist. This is the very issue in question.
2. "If the parts are contingent, then the whole is contingent." Again, an argument (Argument C) may be given to show the contrary:
(1") Contingent beings exist.
(2") Their existence implies the existence of other beings upon whom they depend for their existence.
(3") Because no necessary being exists, the whole cannot be contingent on any such being.
(4") The only alternative is that the whole is non-contingent.
Therefore, it is false that if the parts are contingent, then the whole is contingent.
Like Argument B, Argument C assumes the truth of atheism and is question-begging. But so is premise (5) of Argument A:
(5) If the sum of contingent beings does not have a first member, a necessary, self-existent, unchanging being must exist. Otherwise, the contingency, non-self-existence, and changing nature of the whole would not be explained. ("If the parts are contingent, then the whole is contingent.")
An atheistic universe, all of whose members form an interrelated whole, requires that the whole be non-contingent, as Argument C demonstrates. However, to assume that the sum of contingent parts must also be contingent, as (5) of Argument A does, is to guarantee the theistic conclusion in advance. If the whole is contingent, a necessary being must be its cause. (A contingent cause would simply be part of the whole.) But, again, this is the very issue in dispute. Why not just say that the whole is itself necessary?
In sum, to assume that nothing can come from nothing or that the whole universe is contingent if its parts are contingent is to presuppose the truth of theism. To assume the opposite presupposes the atheistic conclusion.
The cosmological argument, then, doesn't fail because it lacks perspicuity. All in all, it is fairly straightforward and easy to understand. It fails because it assumes the truth of theism. A person who does not already believe that the universe as a whole or its first finite cause is dependent on a necessary being will remain unconvinced.
Although the cosmological argument fails as a proof for God's existence, the real issue is now before us. This is the issue of autonomy versus dependence. The atheist is committed to the autonomy of the universe. Whether he asserts that the first finite being simply came into existence or that the universe as a whole exists on its own, he refuses to acknowledge any dependence on God. In his eyes, the space-time universe is self-sufficient. Indeed, it serves as his god.
In this, he behaves exactly as the Bible says he will. In Romans 1:18-23 the Apostle Paul declares:
For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who hold the truth in unrighteousness; because that which may be known of God is manifest in them; for God hath showed it unto them. For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse: Because that, when they knew God, they glorified him not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened. Professing themselves to be wise, they became fools, and changed the glory of uncorruptible God into an image made like to corruptible man, and to birds, and four-footed beasts, and creeping things.
According to Paul, the problem for the unbeliever is not ignorance. Nor is it the inability to understand the cosmological argument. That argument rightly points out that man and the world are not self-sufficient, that they are dependent on God. The problem is rebellion and a willful disregard for the truth. The unbeliever knows better. (He "holds the truth in unrighteousness.") But he would rather assume that the world simply popped into existence out of nothing or that it somehow is self-sustaining than acknowledge the Creator from Whom he is doing everything he can to hide. That this universe is dependent is clear--so clear that men who do not acknowledge this are "without excuse." But those who are in rebellion against God must assume the contrary, and so long as they do, they will never be persuaded by the cosmological or any other argument for the existence of God.
In reality, the cosmological argument as traditionally presented does not go far enough. Every object, every event, every fact, every thought in this space-time universe is utterly dependent on God for its origin and ongoing existence. As Paul told the philosophers gathered on Mars' hill, "In [God] we live, and move, and have our being" (Acts 17:28). The cosmological argument is designed to show that the first finite cause or the universe as a whole is dependent on God. In truth, however, God's providence and governance extends to everything in this world.
But this is exactly what the unbeliever refuses to admit. No argument, the cosmological or any other, will get him to see it. If he starts with presupposing the autonomy of the universe (which he must do so long as he is in rebellion against God), he will reject at all costs the idea that the universe is dependent on God. Indeed, his atheistic conclusion will appear rational to him. The unbeliever will reject the very starting point of traditional natural theology--the arguments for the existence of God--because he must reject them.
We are now in a better position to evaluate the assumption (P) with which we started. Even the minimal S-proposition, "God exists," is not rationally demonstrable, in the sense intended by advocates of natural theology. By its very nature, it challenges the professed autonomy of the unbeliever. Because this autonomy is foundational for him, he will reject the conclusion "God exists," no matter how carefully arguments to establish this conclusion are drawn. If the minimal S-proposition is not rationally demonstrable, however, then neither are any other S-propositions. Natural theology as conceived by Aquinas and other rationalists cannot even begin to get off the ground.
Rationalistic natural theology makes two crucial strategic blunders. The first is that it treats the unbeliever's problem as though it were primarily intellectual. Although the intellectual aspect cannot be ignored (the mistake of the fideists), the primary problem is sin. The unbeliever is capable of reasoning as rationally as is the believer. He may even possess greater reasoning capacity. He is kept from seeing the truth not through a lack of intellectual acumen, but because his autonomous starting point will not allow him to draw theistic conclusions.
Even more important is the second blunder. Rationalistic natural theology assumes that the believer can share a professedly neutral common ground with the unbeliever. He claims to start with an assumption that does not require belief in a transcendent, sovereign God Who governs every facet of the universe. For those like Thomas, who ground their natural theology on the cosmological argument, that starting point is contingent matter in motion. The existence of God, then, is supposedly derived from this naturalistic starting point. As we have seen, this conclusion follows only if God's existence is surreptitiously presupposed. Although not all unbelievers are able to identify the problem, they implicitly sense a fallacy in this argument and therefore reject it.
By starting with an allegedly neutral fact, the rationalist gives away the ranch. If the sovereign God of the Bible exists, there can be no neutral facts. If neutral facts exist, the God of Christian theism does not exist. Even if the cosmological argument somehow persuaded an unbeliever of the existence of God, his assumption of the autonomy of man would not have been challenged. God would merely be the most plausible hypothesis to explain the existence of the world. The total sovereignty of almighty God would remain hidden from him.
Van Til rightly sums up this approach to natural theology:
They [i.e., all non-presuppositional forms of natural theology] have allowed that...natural man is right, at least to some extent, in asserting his autonomy. If this is true, then the natural man would also be right, to an extent at least, in claiming that he can stand in judgement over the revelation, even the redemptive revelation, of God. 
Although the Christian apologist ought to reject the rationalistic approach, he is not thereby forced into the fideist camp. Fideism is not so much apologetics as capitulation. To claim that Christianity is to be believed despite the fact that it is not rational is to do as much disservice to the God revealed through Scripture as rationalism does. God is the Supreme Mind and the source of all true rationality. The unbeliever's problem is not that he is too rational or that he lacks faith. He does have faith--in the autonomy of man. His problem is that his ability to reason has been thoroughly distorted by the sinful presupposition of man's autonomy which stands in the way of truly thinking aright.
The fideist may regard the apologetics task as hopeless. Not so the presuppositionalist. While he rejects the traditional arguments for the existence of God, he has a better starting point--the self-attesting sovereign God of Scripture. From this ground he can successfully challenge all belief systems which deny this.
The Bible makes clear that God is present everywhere and governing every aspect of creation. In addition to the already-quoted passages (Acts 18:28, Romans 1:18-22), Psalm 139:7-12 tells us:
Whither shall I go from thy spirit? or whither shall I flee from thy presence?
If I ascend up into heaven, thou art there: if I make my bed in hell, behold, thou art there.
If I take the wings of the morning, and dwell in he uttermost parts of the sea;
Even there shall thy hand lead me, and thy right hand shall hold me.
If I say, Surely the darkness shall cover me; even the night shall be light about me.
Yea, the darkness hideth not from thee; but the night shineth as the day: the darkness and the light are both alike to thee.
Any presupposition, such as the belief in autonomy, which denies this inevitably leads to chaos. The unbeliever does not see this because he is unwilling to apply this presupposition consistently. To live with the fairly well-ordered, rational set of beliefs he does, he must tacitly accept certain theistic presuppositions.
For example, as we have seen, his rejection of the cosmological argument leads him to assume a self-existent first finite being or a self-existent universe. This assumption fits in quite well with his desire for an autonomous starting point. Consistency, however, demands that he go much further. If the first finite being is self-existent, other finite beings could possess this property. If the universe as a whole is self-existent, why not its parts? A universe of self-existent beings would be chaotic. The unbeliever, however, does not apply his principle consistently. Rather, once he thinks he has eliminated God from his system, he tacitly accepts a principle of order which is explicable only on the assumption of a Supreme Orderer of the universe.
Similarly, his rejection of belief in a Supreme Lawgiver should lead to his rejection of all morality. The fact that it doesn't, that he still believes in right and wrong, shows that he refuses to apply his principles consistently. The Christian apologist should show him where these principles really lead. (I have not argued for this point. This would take us beyond the scope of this paper. I simply raise it to show how apologetics should proceed in another area.)
At the same time, the Christian apologist should challenge the unbeliever to show whether such confusions arise from a Biblical starting point. Knowing that God is the God of order and not chaos, he should be eager to see this challenge accepted.
The apologist who adopts this starting point is under no illusion that the unbeliever will thereby see his folly. The only hope for the unbeliever is through the awakening and illuminating work of the Holy Spirit. This does not mean, however, that all apologetic methods are equally effective--or equally useless. God uses human means to reach the lost (cf., Romans 10:14ff). He expects us to use the best means at our disposal. For God's Spirit to work, a person's pride must first be abased. An apologetic that destroys the folly of autonomous reasoning will be much more effective to this end than will one which appeals to man's ability to reason apart from God.
In closing, I want to answer two criticisms of presuppositionalism raised by Norman Geisler. Because he erroneously classifies presuppositionalism as a type of fideism,  he includes these in his critique of fideism. It is clear, however, that he has presuppositionalism in mind.
The first is that "fideists [i.e., presuppositionalists] do not differentiate clearly the difference between the basis of belief in God and the support or warrant for that belief....The fideists properly stress the basis for belief, namely, God or his revelation; but they seem to neglect entirely the warrant or support for exercising this belief." 
While this criticism may be applicable to some forms of fideism, it entirely misses the mark if it is directed at presuppositionalism. For the presuppositionalist, every fact, every thought, every experience provides warrant for belief in God. With the Psalmist he exclaims, "The heavens declare the glory of God; and the firmament showeth his handiwork" (Ps. 19:1). Even the cosmological argument, the linchpin of the rationalist's warrant, has its place. Rightly understood it shows the utter dependence of this world on a self-sufficient Being and the consequences of denying this truth. This warrant, then, is far broader than the warrant of the rationalist. "In him we live, and move, and have our being."
Geisler's second criticism is that "fideists fail to understand the implications of the difference between the unavoidability of and the justifiability of presuppositions...The crucial question is not whether we can avoid using presuppositions but whether we can justify those we use."  Geisler apparently believes that the presuppositionalist has no way of justifying his presuppositions.
To the contrary, he does. His justification is this: Only the theistic view makes sense of all of reality. And insofar as the unbeliever can justify his beliefs he must rely upon theistic assumptions to do so. In a word, everything is ultimately a justification of his theistic presuppositions. One can't have better justification than this. As Van Til has noted: "The natural theologian [i.e., the rationalist] is like a child who daily and constantly sees his father in the father's home and yet who takes a lantern in order to search for him."  The presuppositionalist, on the other hand, knows that all men walk daily in the light of God's presence. Therefore, he stands with confidence on this ground, trusting that God will graciously shed His light into the mind and the heart of the unbeliever.
The attempts by modern evangelicals to resurrect Thomism have failed. Its very starting point--the cosmological argument--does not accomplish its purpose. It is now time to bury the corpse once for all.