The goal of presuppositional apologetics, or at least some versions of it as I understand them, is to prove the necessity of the Christian worldview: Christianity is true and only Christianity, and all others are incoherent and inconsistent and cannot account for reality. Christianity, also, is necessarily true, or so they say. Now for a thing to be necessarily true is two things, though they both basically are the same: it must be true in every possible world, and its denial must entail a contradiction. There are problems with the claim that Christianity is necessarily true, one of them being the fact that no Christian presuppositionalist has shown that at least all the other worldviews we know about are inconsistent and contradictory, let alone all possible worldviews which are not Christianity. The difficulty of that sort of task aside, however, it can easily be shown that Christianity is not necessarily true: Imagine a possible world where God creates man and never decrees that man sins (or, if you don't believe that God decrees sin, imagine a world where he creates men without free will and they always do good). In such a world, there is no incarnation and no atonement, because there is no need, because there is neither of these things, there is no Christianity. Christianity is not true in this worldview because there is no sin and no Jesus of Nazareth who took on the sins of the world. So Christianity is not necessarily true. A presuppositionalist can defend his claim to the necessary truth of Christianity by supposing that such a world is not actually possible--though it's hard to see how that might be the case. It certainly seems to be free of contradiction and absurdity. (Unless the presuppositionalist wants to claim that, because of God's nature, he decrees that men sin necessarily--but that is a mighty strange claim and doesn't seem to be very plausible.) What does the presuppositionalist argue for, then? For the necessary existence of the Christian God? Even if he can argue for that (even though it seems troublesome and difficult enough), that doesn't prove Christianity true at all. He can exist and Christianity still be false, as was shown above. Then the presuppositionalist is at best arguing for a form of theism and not Christianity, which was his purpose from the beginning. What can the presuppositionalist do about all this? Perhaps he ought to adopt a more liberal approach, distancing himself from the fundamentalism of Cornelius Van Til and Greg Bahnsen, arguing merely that presupposing Christianity makes a great deal of sense about the universe, a sort of abductive argument for Christianity. This I have no problems with, and it is quite a quick-and-useful method when discussing the truthfulness of the Christian faith, but it is some distance from what the first presuppositionalists set out to do.