Some conceptions of antithesis can unwittingly work to undermine the very antithesis which is essential to the Biblical viewpoint.
The very idea of apologetics (defending the Christian faith against its cultured despisers) calls for a healthy and sound notion of antithesis, or else apologetics makes no sense. Since belief and unbelief ultimately stand over against each other, they need to challenge each other or call each other to account, as though the stakes in their dispute were eternal.
Indeed, any genuinely faithful and effective work of Christian scholarship, preaching, missions, or apologetics requires the believer to be aware of, and to work in the guiding light of, the pervasive Biblical message of antithesis. And this summons to proclaim and apply this antithesis in the pursuit of our ministries is especially urgent in our modern day of intellectual relativism, antipathy to distinctively Christian methods and conclusions, and theological compromise regarding the doctrine, discipline and worship of Christ's church.
One might think then that we would welcome any Christian scholar or writer who makes the summons back to antithesis central to his encounter with modern culture. But this is not entirely the case. In a rather odd way, some conceptions of the antithesis can unwittingly, but nevertheless truly, work to undermine the very antithesis which is presented in, and essential to, the Biblical viewpoint. That is, a false or misconstrued conception of the necessary antithesis could actually have the effect of hampering Christian apologetics by denying the true antithesis which sorely needs recognition in our day.
I believe that this is what we find in the case of Francis Schaeffer's apologetical work and writings. Those familiar with the popular and prolific publications which came from the pen of our dear brother, Dr. Schaeffer, cannot easily forget his polemic in favor of antithetical thinking. At the back of The God Who is There Schaeffer defined the word "antithesis" very simply as "direct opposition or contrast between two things." We also find him using the word loosely for a movement from one condition to its opposite, as when he calls, "the point of conversion, when the individual passes from death to life...the point of personal antithesis." However Schaeffer's more characteristic and basic use of the concept of "antithesis" is associated with themes which are pivotal in his defense of the faith, themes like:
(1) the claim that knowledge precedes faith thus opening the door to a "pre-evangelism" based, not on Scriptural truth or authority, but on the unbeliever's awareness of the form of the external universe and the mannishness of man,Schaeffer became famous (and I think infamous in some circles) for his sweeping surveys and evaluations of the history of philosophy and of the arts, the gist of which was definitively set down in his book, How Should We Then Live?, subtitled "The Rise and Decline of Western Thought and Culture."
(2) the requirement that any proof -- whether in science, philosophy, or religion -- show that a theory is non-contradictory, that it explains the phenomena in question, and that it can be lived consistently,
(3) the treatment of Christian and non-Christian presuppositions simply as hypotheses which are judged by the test of which set "fits the facts,"
(4) a commendable emphasis on the non-Christian's inability to live logically with his presuppositions, especially
(5) the notion of a cultural crisis whereby modern man has been forced to pass "below the line of despair."
Before his [Hegel's] time truth was conceived on the basis of antithesis....Truth, in the sense of antithesis, is related to the idea of cause and effect. Cause and effect produces a chain reaction which goes on in a horizontal line. With the coming of Hegel, all this changed....[Hegel proposed] from now on let us think in this way; instead of thinking in terms of cause and effect, what we really have is a thesis, and opposite is an antithesis, and the answer to their relationship is not in the horizontal movement of cause and effect, but the answer is always synthesis....[Thus] instead of antithesis we have, as modern man's approach to truth, synthesis.In How Should We Then Live, Schaeffer contends that "non-Christian philosophers from the time of the Greeks until just before our modern period" took reason seriously. They accepted the validity of reason, that the mind thinks in terms of antithesis. The first lessons in classical logic were: A is A, and A is not not-A." But then, according to Schaeffer, there came a "shift" in philosophy, a shift toward romanticism and with it a pessimism regarding rationality. This shift was the responsibility, he says, of Rousseau, Kant, Hegel and Kierkegaard. And it created, he thinks, the characteristic "mark of modern man," namely that the lower realm of reason is taken to lead inevitably to despair and is totally separated from the upper realm of values, meaning, and optimism. 
Hegel argued that antithesis has never turned out well on a rationalistic basis, so he proposed to change the methodology of epistemology. Instead of dealing with antithesis, let us deal with synthesis. So he set up his famous triangle -- everything is a thesis, it sets up an antithesis, and the answer is always a synthesis....He [thus] changed the whole theory of how we know.
What did Hegel say? He argued that attempts had been made for thousands of years to find an answer on the basis of antithesis and they had not come to anything. Philosophic humanistic thought had tried to hang on to rationalism, rationality, and a unified field [of knowledge], and it had not succeeded. Thus, he said, we must try a new suggestion....He changed the rules of the game in two areas: epistemology, the theory of knowledge and the limits and validity of knowledge; and methodology, the method by which we approach the question of truth and knowing.
What he said was this. Let us no longer think in terms of antithesis. Let us rather think in terms of thesis-antithesis, with the answer always being synthesis....[Schaeffer comments that] a choice was made, and the choice consisted in holding on to rationalism at the expense of rationality.
When Schaeffer addresses Hegel's role in this development, he mentions the dialectical unfolding of history whereby successively new syntheses are formed through the perception that "there is truth in both thesis and antithesis." Schaeffer concludes that although it is "an over-simplification of Hegel's complete position, the result is that all possible particular positions are indeed relativized....This has lead to the idea that truth is to be sought in synthesis rather than antithesis."  What we find in the major works of Francis Schaeffer, then, is a conspicuous insistence upon, and call for thinking in terms of "antithesis." The Christian's witness to his modern culture and the apologetical solution that Schaeffer offers to our culture's philosophical despair involves the proclamation of "antithesis" over against the relativizing tendency of synthesis-thinking in Hegel and the post-Hegelian philosophers. Given our earlier discussion of the Biblical view of antithesis (Antithesis Vol. 1, No. 1, 1990) between believer and unbeliever, as well as the disastrous consequences of modern culture's suppression of that antithesis, can we now welcome and endorse the position of Schaeffer on the subject of antithesis? Can we eagerly list the works of Schaeffer in our project of restoring a consciousness of "the antithesis" to contemporary Christian scholarship? Sadly, we cannot do so at all.
Schaeffer's desired antithesis simply substitutes an older version of humanistic thought for a newer one. In so doing, it is not true to the Biblical antithesis itself. Paul could challenge the philosophical Greeks of his day -- the day of Schaeffer's desired antithetical reasoning -- saying, "Where is the wise, where is the disputer of this age? Has not God made foolish the wisdom of this world?" (I Cor. 1:20). The development and demonstration of the foolishness of apostate thought did not await the advent of Hegel. The despair of unbelieving philosophy was just as clear in the days of ancient Greek speculation.
Hegel's proposals were not on this order whatsoever. Hegel did not berate logical consistency; indeed, he forcefully advocated a coherence view of truth! What Hegel deemed inadequate was finite man's conceptualizing of reality and giving it rational expression. In the preface to Phenomenology of Mind, Hegel said that "the truth is the whole." Therefore, any set of propositions which falls short of being a complete system covering the whole of reality will, due to its incompletion, generate its own inadequacies or contradictions. The categories we use in our logical thinking are not definitive according to Hegel, but rather tentative and provisional. Between a thesis and its antithesis there will be a dynamic tension because both positions contain something which is rational in them, yet both prove to be inadequate in themselves. The whole point of coming to a synthetic resolution of that tension is not to renounce logical consistency, but precisely to preserve what is rational in both positions and cancel out what is not rational! All contradictions will be reconciled and all falsity removed in the dialectical unfolding of our thinking only when our system of thought is complete -- at which point it will have developed into a complete unity with the object or reality which is the content of our thought.
Hegel's dialectical method does not view the world as a collection of externally related, discrete things; it rather encourages us to understand reality as an evolving process. As such, Hegel's philosophy warns us against taking any given stage of the developmental process (in thought or in history) as fully adequate. Such a stage is not wholly mistaken, but it is nevertheless only a misleading representation of what shall ultimately develop from it (as a tadpole stands to the frog). This philosophical perspective may or may not be radically muddle-headed, and it may or may not be overly humble about finite man's ability to conceptualize ultimate truth; but it is still miles from being the renunciation of logical validity and rational thinking which Schaeffer paints it to be.
Our bottom line assessment of Schaeffer's plea for renewal of antithesis over against the synthesis of modern thought is that he misconstrued the Hegelian thought which he heatedly opposed and, at the same time, overlooked the true nature of that antithetical thinking to which God's word calls us as Christians.
Our challenge should be for Greeks and Hegelians alike to make sense of their use of logical laws, given their presuppositions about reality, man and knowledge. We should show them that given their perspectives on life and thought, the existence and the normativity of abstract logical principles is just unintelligible. Their rationalism in philosophical speculation does not conform to their presupposed irrationalism about the world.
Given their worldview, then, unbelievers cannot justify the most elementary laws of thought (even those like the Barbara syllogism) which non-Christians and Christians use alike. Thinking themselves to be wise, they have in principle become fools instead. The vanity of unbelieving philosophical thought, as presented in the Bible, is not the exclusive province of ancient or modern culture. It is the result of that primal enmity which stands between God (or His people) and all apostate cultures indiscriminately.
According to Schaeffer, the philosophical challenge issued by the apologist does not pertain to the natural order, for unregenerate man can make sense of his world scientifically and logically as far as he goes. As a result, Schaeffer claims that the apologist is supposed to show is that there is "more to reality" -- another realm, another dimension -- which the unbeliever's thinking has not touched. What is wrong with the non-Christian perspective on the physical world is not that it is, in principle, unintelligible, but simply that it is incomplete. And thus Schaeffer says to the materialist philosopher or scientist after he has expounded what he knows about the universe:
This is all very fine [the non-Christian outlook], but it's drastically incomplete. It's as if you had taken an orange, sliced it in half, and only concerned yourself with one of the halves. To really understand reality in our universe you have to consider both halves -- both the seen and the unseen....You are completely unbalanced. You only know half of your own universe.Schaeffer then mysteriously comments that between the views of naturalism and supernaturalism there is "a total antithesis"; the two "can never be brought into synthesis. " But on Schaeffer's conception of the difference between believing and unbelieving thought, there is little reason at all that they could not be synthesized. The difference between them is virtually quantitative, not qualitative. The unbeliever merely does not have "both halves" of the orange, and what he says about the half he does have is "all very fine," according to Schaeffer's portrayal.
Though some who are justly fond of Schaeffer for his personal ministry might claim that this is a minor inconsistency (an unfortunate illustration), this sort of "mistake" is not a minor matter for apologetics. Getting things so wrong on this particular point would be akin to a medical doctor making the "minor error" of confusing his patient's lungs and legs. Moreover, this unfortunate illustration is not at all uncharacteristic of what Schaeffer repeatedly says elsewhere. I would have you take for instance his notion that believers and unbelievers alike have a presupposition-less understanding of "the facts" against which they may judge the respective adequacy of the Christian and non-Christian "presuppositions." There is apparently no antithesis for Schaeffer when it comes to regenerate and unregenerate minds understanding the facts.
So then, Schaeffer's conception of the apologetical situation was neither calculated nor suited to present a bold challenge to unregenerate thought in the realm of science or logic (the lower "half of the orange," if you will). And the reason why it was not is that such a conception does not understand or appreciate the true nature of the deep-seated and all-pervasive antithesis between the Christian and non-Christian worldviews. The antithesis presented in Scripture and which must be respected by the apologist actually operates at a far more fundamental level than what Schaeffer himself had seen. The true antithesis is of entirely different order than Schaeffer thought.
My discussion of Dr. Schaeffer has not intended to take anything away from our appreciation of the man and what he accomplished by God's grace. Rather, it has aimed to help us to understand that the antithesis revealed in the Bible operates between believers and unbelievers at the level of their basic worldviews -- not merely in their opinions about logical consistency or about "the other half of the orange" (the unseen universe). Hopefully, we can now accurately perceive and philosophically appreciate this weighty truth by setting it in contrast to Dr. Schaeffer's overly light handling of it. After all, the very idea of apologetics calls for a healthy and sound notion of antithesis, or else apologetics makes no sense.
 (Chicago: InterVarsity Press, 1968).
 Ibid. p. 177
 Ibid. p. 130
 Ibid. pp. 111,121, 129, 142, 143
 Ibid. p. 109
 He is There and He is Not Silent, (Wheaton: Tyndale House, 1972), pp. 65, 66, 81.
 God Who is There, pp. 121-127
 Ibid., passim; and Escape From Reason (Chicago: Inter-Varsity, 1968).
 (New Jersey: Fleming H. Revell, 1976).
 God Who is There, p. 20
 He is There, p. 45
 God Who is There, p. 20
 He is There, pp. 45-46
 Escape From Reason, pp. 40-41
 pp. 145, 146
 Ibid. p. 151
 Ibid. pp. 152-163
 Ibid. p. 164
 Ibid. p. 162 -- according to Schaeffer's citation of James Sire's summary of Copleston's summary of Hegel!
 Ibid. p. 163
 Death in the City, (Chicago: Inter-Varsity, 1969) pp. 129, 130, 131
 Ibid. p.131
 He is There, pp. 65,66,81 -- "Presupposition" has come to mean little more than "hypothesis" for Schaeffer.