Ok, willie.  Here's the deal with pressupositionalism:

The most obvious problem with presupposition is that it is a circular argument.  That is, it assumes its conclusion to be true.  This is a logical fallacy.

Presuppositionalists try to get around this problem by asserting that knowledge of the Christian god is a priori, or self evident and removed from experience.  What they're essentially trying to do is say that knowledge of the Christian god is axiomatic.

This can be proven false very simply.  Axioms, after all, are things which are true because there is no other possibility.  Consider the axiom of identity, made famous by Descartes.  If we question our own existence, our existence must be self evident.  It is impossible to imagine an alternative to existing once one has questioned his own existence.

So, let's examine the Christian God and ask, "Can we imagine any alternative which is plausible?"  If yes, then the Christian God is not self evident.  Here, I'll try one:  I can imagine that Allah is the one true god.

Done.  The God of the Bible is not self evident because it is, at the very least, plausible that Allah is the one true god, and that through some set of circumstances, everyone has been wrong about the Christian God.

Some presuppositionalists will try to argue that the Christian god, per se, is not self evident, but that "God" is.  This leads us down the path of all the fallacies we see so often -- the argument from incredulity, argument from ignorance, circular reasoning, non-sequitur, ad nauseum.

Even so, this approach fails on a more basic level.  ANY god that I can imagine admits an alternative.  That is, no matter how impressive sounding I make my proposed god, someone else can come along and say, "That's not the only way it could happen."  It's simply not self evident.

Finally, presupposition fails even more devastatingly.  Look carefully at the first quote I gave you about presuppositioin.  What is it?

Quite obviously, it's a premise.  A proposition.

What, exactly, are premises and propositions?  They are elements of logic.  The thing is, the premise attempts to discount logic!  It says, "a Christian cannot consistently declare his belief in the necessary existence of the God of the Bible and simultaneously argue on the basis of a different set of assumptions in which God may or may not exist..."  That other system, of course, is logic.  Since God is not axiomatic (and this can be demonstrated with 100% certainty... I just did it, after all) then under the system of logic, the only alternative is that either God exists or he doesn't.

So... the argument for presuppositionalism IS LOGIC, though in the same sentence, it has been claimed that logic is incompatible with knowledge of god!

Presuppositionalism literally self-pwns itself.  It is inherently contradictory while using the very thing it attempts to discredit.