With respect to this matter of non-existence, it would seem then that four theoretical possibilities are open. There may be those (a) who think it reasonable to doubt the existence of God but unreasonable to think of the non-existence of the universe. There may be those (b) who think it possible to think intelligibly of the non-existence of both God and the universe. There may be those (c) who think it impossible to think intelligibly of the non-existence of either the universe or of God. Finally, there may be those (d) who think it possible to think intelligibly of the nonexistence of the universe but impossible to think intelligibly of the nonexistence of God.
Of these various possibilities it will at once be observed that the acceptance of any of the first three positions puts one on the antitheistic side of the argument. Only the last position is consistent with theism. But it will also be observed that in many instances any one of the first three positions is taken for granted at the beginning of an argument without awareness of the fact that those holding the position have therewith foreclosed to themselves the possibility of arriving at a theistic conclusion. In other words, any one of these three positions is thought to be consistent with the application of a strictly empirical method of research which, it is thought, may lead to any conclusion whatsoever.
Cornelius Van Til, A Survey of Christian Epistemology.