Here is a succinct answer to a question I was asked about presuppositionalism and its relation to God's existence:
//Am I correct in thinking "presuppositional apologetics" begins on the foundation of God's being and doesn't give ground to those that would say "Let's suppose for a moment that God doesn't exist"?//
Presuppositional apologetics is simply an admission that apologetics is predicated upon the assumption of [a] first principle(s). If I ask you why you believe a proposition, you will probably give me a reason. What if I ask you why you believe that reason? You may give me another reason. Consider what would happen if I kept questioning your premises. We quickly see that there must be a proposition which is taken for granted, a proposition upon which all knowledge within a given epistemological system is deduced. Otherwise, we have no way of knowing anything, as I could ask you for external justification of your premises ad infinitum.
Christian presuppositionalists sometimes confuse metaphysics with epistemology. Metaphysics deals with ontology, epistemology deals with knowledge. Obviously, we can't know what exists if we can't justify what we know, so epistemology should take precedence. Hence, when some Christian presuppositionalists begin with "God exists," too many questions are begged for which the first principle cannot answer: which God? By what means have we discerned this? How can we understand God? Etc.
Hence, I believe that Christian presuppositionalism begins, not with God, but with God's revelation: the Bible is the sole, extant source of knowledge. From Scripture, we deduce everything we know, including God's existence and character.