Tim Keller for The Gospel Coalition:
You can’t doubt belief A except on the basis of some belief B you’re believing instead at the moment. So, for example, you cannot say, “No one can know enough to be certain about God and religion,” without assuming at that moment that you know enough about the nature of religious knowledge to be certain about that. Doubt your doubts.
Good, quick read where Keller cites and responds to five common “occasions for doubt.”