By this rejection of God, agnosticism has embraced complete relativism. Yet this relativism must furnish a basis for the rejection of the absolute. Accordingly, the standard of self-contradiction taken for granted by antitheistic thought presupposes the absolute for its operation. Antitheism presupposes theism. One must stand upon the solid ground of theism to be an effective antitheist.
(Cornelius Van Til, A Survey of Christian Epistemology, p. xi)
“Antitheism presupposes theism” is one of Van Til’s best lines, because it captures in a nutshell the genius of presuppositional apologetics. It’s not merely that theism is true; it’s not merely that theism can be shown to be true; it’s that theism can be shown to be true by any attempt to prove it false. One can prove theism to be false only if, as a matter of fundamental metaphysical fact, theism is true — which is just to say that antitheism is self-defeating.