There are two ways of constructing a proof for the existence of God. These two ways are mutually exclusive. The one is in accord with the basic construction of Reformed theology; the other is destructive of it. The one begins with the presupposition of the existence of the triune God of the Scriptures. The other begins with the presupposition of man as ultimate.
The true theistic proofs undertake to show that the ideas of existence (ontological proof), of cause (cosmological proof), and purpose (teleological proof) are meaningless unless they presuppose the existence of God.
This involves interpreting human reason itself in terms of God. It involves saying that unless human reason regards itself as being what Scripture says it is, created in the image of God, that then it has no internal coherence.
– Cornelius Van Til, Common Grace and the Gospel p.218