The trinitarian believes God is Triunity. That is a belief gleaned from the Bible. Therefore, he also believes the Bible to be true.
This stands as the watershed presupposition. If the Bible is not true, then trinitarianism is untrue and Jesus Christ is not who He claimed to be. We learn nothing about the Trinity or Christ from nature or from the human mind. And we cannot be certain that what we learn from the Bible about the Triune God is accurate unless we believe that our source itself is accurate. Thus the belief in the truthfulness of the Bible is the basic presupposition. This will be fully discussed under inspiration and inerrancy. (Charles Ryrie, “Basic Theology,” 16)
According to Ryrie the ground of truthfulness really is a priori in our mind — even though he says this is not true. In other words, before we get to the Triune God; we first must be “certain” of something ourselves, and then we are able to have knowledge of the Christian God. But is this really the procedure we want to follow? Is this really the order of knowledge that is basic? Does scripture come before Jesus, or does it come after Him; or both?
As much as I still appreciate Ryrie — given my heritage — there most certainly are tinges of rationalism throughout his theologizing; wherein he puts too much weight on the human intellect as the ground for everything (reminds me of our dear friends Thomas Aquinas and Rene’ Descartes — at least in re. to their intellectualist anthropology).
One thing, once again that I do agree with Ryrie on is that Scripture serves as the deposit and source for how we know the Triune God; nevertheless, I think Ryrie should frame this differently (not make Christianity a “rationalist” faith).