In fact, no non-Christian can be consistent to the logic of his presuppositions. The reason for this is simply that a man must live in reality…. No matter what a man may believe, he cannot change the reality of what is. As Christianity is the truth of what is there, to deny this, on the basis of another system, is to stray from the real world.
— Francis Schaeffer, from The God Who Is There, p. 121