Paul Tillich argued that the Scripture cannot be used as proof that Christianity is the absolute and/or universal religion. "Proof texts" are acceptable only among Christians and this is not acceptable criteria to the secular minds or to the perspective of other religions. Thus, the absoluteness of Christianity cannot be proven theoritically. Below is an interesting quote from Paul Tillich about the importance of missionary activity in support of a very important theological issue.
There was a discussion, especially in the last period of liberal theology, about the absoluteness of Christianity. Is Christianity the absolute religion? Is Christ the center of history? Is he the bringer of the New Being? Or are the other religions of equal value and does each culture have its own proper religion? Christianity, according to these ideas, belongs to the Western world, and it should not interfere with the religious developments of the Eastern world. This, of course, would deny the claim that Jesus is the Christ, the bringer of the New Being. It would make this statement obsolete, because he who brings the New Being is not a relative figure but an absolute figure of an all-embracing character. The New Being is one, as being itself is one.
This universality of the Christian message, its universal claim, includes what has been called, with a not too happy term, the "absoluteness of Christianity." Let me call it its universality. Now, how can you prove, today, as a Christian, or as a theologian, that the Christian message is universal and valid for all cultures and religions, so that Christ must become what he potentially is, the center of history for historical developments? How can you prove this? The answer obviously is: you cannot prove it at all in terms of a theoretical analysis, for the criteria used in order to prove that Christianity is universal are themselves taken from Christianity. Therefore, they do not prove anything except for those who are in the Christian circle. This means: there is no theoretical argument which can give the proof of the universality of Christianity and the claim that Jesus is the Christ. Only missions can provide that proof.
Missionary work is that work in which the potential universality of Christianity becomes evident day by day, in which the universality is actualized with every new success of the missionary endeavor. The action of missions gives the pragmatic proof of the universality of Christianity. It is a pragmatic proof. It is the proof, as the Bible calls it, of power and Spirit. It is not a theoretical proof, which you can give sitting in your chair and looking at history; but, if you are in the historical situation in which missions are, then you offer a proof, a proof which is never finished. The element of faith is always present, and faith is a risk. But a risk must be justified, and that is what missions does. It shows that Jesus as the Christ and the New Being in him has the power to conquer the world. In conquering the world, missions is the continuous pragmatic test of the universality of the Christ, of the truth of the Christian assertion that Jesus is the Christ.