“With such as these it would seem that the point we should be most anxious to drive home is that in trying to be agnostic, and in trying to say that they have no need of metaphysics, they have already given one of the two possible answers to every question of epistemology that may be asked. They have, as a matter of fact, said that all the facts—or, in epistemological language, they have said that the object and the subject of knowledge—exist apart from God and are able to get along without God. They think they have said nothing at all about ultimate matters, while as a matter of fact they have in effect said everything that could be said about them, and, we believe, more beside. They have tried to be so modest that they did not dare to make a positive statement about anything ultimate, while they have made a universal negative statement about the most ultimate consideration that faces the mind of man. That this charge is fair is apparent from the consideration of the opposite. Suppose that the object and the subject of knowledge do not exist apart from God. Suppose, in other words, that the Christian theistic conception of philosophy is true. In that case, it is not only possible to know something about ultimate things, but in that case the knowledge of proximate things depends upon the knowledge of ultimate things. In that case, not a single fact can be known unless God is known.
What the present-day agnostic should do then is to make his position reasonable by showing that God does not exist. The burden of the proof is upon him. He claims, of course, that the burden of the proof is upon us when we hold that God exists. Yet this is clearly not the case, since his own position, to be reasonable, must presuppose the non-existence of God. If God does exist, man can know him, for the simple reason that in that case all knowledge depends upon him. Hence an agnostic position must first prove that God does not exist.
From these considerations it follows that agnosticism is completely self-contradictory. And it is self-contradictory not only upon the assumption of the truth of theism, but it is self-contradictory upon the assumption of the truth of antitheism, which is the assumption of agnosticism.”
– Cornelius Van Til, A Survey of Christian Epistemology p.212-213