All Presuppositionalists are Foundationalists, but not all Foundationalists are Presuppositionalists.
Ultimately, Presuppositionalism fails because it essentially asserts that the foundation of knowledge is assumption; not just any assumption, but the correct one, which is to say, God.
It fails exactly because suppositions are by definition, baseless, and consequently then, God is no better or worse than any other supposition, but the Presuppositionalist really means that the ultimate foundational belief isn’t a presumption at all but instead is a warranted belief, a justified belief, an objective one.
So if in fact there is a justifiable, foundational belief, then “God is it” also falls prey to Foundationalism’s tendency toward infinite regressions of justification.
“God is it” turns out to be meaningless because being a justifiable proposition, it begs the question, why?
As it turns out, no theory of knowledge requires a foundation. What matters is that our beliefs have warrant. Warrant simply means that we are entitled to some beliefs, and others require a clear line of thinking leading to the concluded belief.
For instance, if one suggests experience is the foundation of knowledge, then preceptions give rise to warrant, not our ability to explain why we believe what we do. So, when we consider other minds, our own existence, the reliability of reason and our reasoning, and so on, these are not presuppositions. They all have warrant, to the extent that suggesting we merely suppose them isn’t just absurd, but unjustifiable; just because we could be mistaken isn’t grounds to suggest we are, and we aren’t merely assuming these things.
One may say that “you are presupposing because you don’t know and can’t prove these things are true!”
This person, however, has lost contact with the reality of what these terms all mean. For example, that I exist is warranted rather than presumed. Warrant is either from justification or entitlements. Knowledge is in the broadest sense, “things that we believe and can’t yet doubt”. In other words, warranted beliefs.
So, I know I exist, I know there are other minds, I know tomorrow will be like today was like yesterday, and so on.
I do not suppose these things, I do not even suppose the truth of these things; because truth is indistinguishable from warrant.
Just a thought.