No human being can escape making an assumption about the nature of possibility at the outset of his investigation. All men have a priori assumptions in terms of which they approach the facts that confront them. The Christian frankly admits that his a priori is the assumption of the existence of the ontological Trinity, the temporal fiat creation of the universe, and man’s creation in the image of God. The non-Christian has a different sort of a priori. Every non-Christian has an a priori. And the a priori of every non-Christian is different, radically different, from that of the Christian.
– Cornelius Van Til, An Introduction to Systematic Theology p.198