It is common knowledge that the presence of children of God in heterodox churches is urged to prove that it is right, even demanded by charity, to fellowship heterodox churches. This is the exact opposite of what Scripture teaches, for Scripture says, "Avoid them." (Rom. 16:17, 1 Tim. 6:3 ff.; 2 John 10-11, etc.)
The argument of unionists is contrary even to natural reason. The old Lutheran teachers point to 2 Sam. 15:11 as an illustration. Just as the fact that two hundred citizens of Jerusalem in their igonorance joined Absalom did not give the rest of Israel the right to desert their king and join the rebels, nor even to connive at the rebellion, so the circumstance that some Christians, from ignorance and contrary to God's orders, follow false teachers does not give license to other Christians to do the same thing.
To say that love demands such a practice is a misuse of that word. Love of God and love of the brethren rather requires the opposite practice. He who loves Christ loves Christ's Word, and Christ commands us to avoid all who teach anything that is contrary to His Word. And whoever really loves the brethren refuses to participate in their erring and sinning, seeking rather to deliver them from error and sin.
Moreover, the Scriptures of both the Old and the New Testament state explicitly that God permits false teachers to arise in order that Christians may show their obedience by avoiding them, not in order that Christians may fraternize with them (Deut. 13:3; 8:2; 1 Cor. 11:19). If Christians, against the divine prohibition, fellowship false teachers and tolerate false doctrines, they commit the sin which the Church calls "unionism," syncretism."
As a matter of fact, this unionism divides the Church and gives rise to heterodox churches in Christendom. Had the Christians always obeyed the divine order to avoid those teaching another doctrine, neither the Papacy nor other sects could have arisen. Where there are no buyers, there is no market. Of course, unionism avers that it aims at the removal of discord among Christians. But because the unity of the Christian Church consists in having one faith and one profession, unionism acutally is a caricature, indeed, a mockery of Christian unity. Instead of healing the hurt, it makes it permanent.
In 1 Cor. 1:10 we have an exact definition of Christian unity: "Now, I beseech you, brethren, by the name of our Lord Jesus Christ, that ye all speak the same thing and that there be no divisions among you." This is a demand for uniformity in speech or in the profession of the Christian doctrine. Then the Apostle continues: "But that ye be perfectly joined together in the same mind and in the same judgment." Here he clearly says that Christians are to use the same words also in the same sense. Agreement in words with disagreement in meaning is altogether contrary to the unity God calls for, and to seek such a "unity" ("we agree to disagree") is immoral, a trifling with sacred, divine things, which is unseemly for Christians.
The Christian Church can and should have patience with the erring and seek through instruction to remove the error. But never can or should the Church grant error equal right with the truth. If it does, it renounces the truth itself. It is the very nature of truth to antagonize error. Truth which no longer excludes error, but grants it domicile, is eo ipso resigning as truth. Pertinently Luther remarks: "Whoever really regards his doctrine, faith, and confession as true, right, and certain cannot remain in the same stall with such as teach or adhere to false doctrine." Unionism in principle abolishes the difference between truth and error, so that only through a "happy inconsistency" can the erring retain their hold on the essential truth. For this reason unionism is a grave threat to the Christian Church. A person is fortunate indeed if, ignoring the words of scripture enjoining him to avoid those who teach another Gospel, he yet retains his faith in the words: "The blood of Jesus Christ, His Son cleanseth us from all sin."
As for the talk current in our day, even among so-called conservative theologians, that "various trends," that is, variations in doctrine and profession, are intended by God, one can merely say that it is shocking that within Christendom the testimony of Scripture should thus be contradicted.
By the term "schism" we mean a divsion in the Church which God's Word does not enjoin, but which is begun by men for carnal reasons and therefore is sinful, e.g., a separation because of differences in church customs, church terms, order of worship, etc. In practice it is important to distinguish between schismatics acting from spite and schismatics acting from weakness in Christian knowledge and prejudice.
Such, however, as separate from a church body because it tenaciously clings to false doctrine are unjustly called schismatics, separatists, etc. This separation is commanded in Scripture (Rom. 16:17) and is the only means of restoring and maintaining the true unity in the Christian Church.