Tim Keller on Doubt and Doubting Doubts
But even as believers should learn to look for reasons behind their faith, skeptics must learn to look for a type of faith hidden within their reasoning. All doubts, however skeptical and cynical they may seem, are really a set of alternate beliefs. You cannot doubt Belief A except from a position of faith in Belief B. For example, if you doubt Christianity because "There can't be just one true religion," you must recognize that this statement is itself an act of faith. No one can prove it empirically, and it is not a universal truth that everyone accepts. If you went to the Middle East and said, "There can't be just one true religion," nearly everyone would say, "Why not?" The reason you doubt Christianity's Belief A is because you hold unprovable Belief B. Every doubt, therefore, is based on a leap of faith.
... The only way to doubt Christianity rightly and fairly is to discern the alternate belief under each of your doubts and then ask yourself what reasons you have for believing it. How do you know your belief is true? It would be inconsistent to require more justification for Christian belief than you do for your own, but that is frequently what happens. In fairness, you must doubt your doubts. My thesis is that if you come to recognize the beliefs on which your doubts about Christianity are based, and if you seek as much proof for those beliefs as you seek from Christians for theirs — you will discover that your doubts are not as solid as they first appeared.