God is the conclusion of Presuppositionalism and not the Presupposition. The Presupposition of Christian Presuppositionalism is Nihilism.
The fundamental flaw and trick of Christian Presuppositionalism is that it hides its true presupposition in order to disguise its conclusion as the presupposition, thus hiding the fact that the argument is inherently self-defeating as the entire argument is predicated on the assumption that human reasoning in a material universe cannot be trusted in order to come to the conclusion that human reasoning can be trusted, thus contradicting the very presupposition that leads to its conclusion.
One must treat logic as self-evidently true and reliable in order to make the Presuppositionalist argument whilst presupposing logic's unreliability. That fact negates the presupposition which the argument relies on and pulls the rug out from under it. We have no need to justify reasoning because we have already affirmed it by trying to justify it.
However, if it is the case that we have reason to consider human reasoning as unreliable then there is no reasoning out of that hole. The moment you affirm "we have reason to doubt human reasoning" you have denied the very basis upon which you make that claim. Any ontological claim is reliant on the coherence of the semantics and epistemology you deploy and Presuppositionalism relies on a fundamentally incoherent starting point to get itself off of the ground. If we have reason to doubt human reasoning, then we have no reason to trust the Presuppositionalist argument no matter how sound it is. If we do not have reason to doubt human reasoning, then the Presuppositionalist argument fails to get off of the ground.
What it comes down to is that we all typically presuppose and affirm logic as tautologically reliable and doing so does not entail the conclusion that God exists. To even get the argument that would lead to that conclusion, one must presuppose a contradiction and then hide that presupposition in order to justify the conclusion. So Christian Presuppositionalism is fundamentally and inescapably incoherent.