Cornelius Van Til

Agnosticism is epistemologically self-contradictory on its own assumptions because its claim to make no assertion about ultimate reality rests upon a most comprehensive assertion about ultimate reality.

The only proof for the existence of God is that without God you couldn't prove anything.

To admit one's own presuppositions and to point out the presuppositions of others is therefore to maintain that all reasoning is, in the nature of the case, circular reasoning. The starting-point, the method, and the conclusion are always involved in one another.