Tim Keller on Doubt and Doubting DoubtsThe Reason for God: Belief in an Age of Skepticism (Dutton: Feb 2008), pp. xvi-xvii.
A faith without some doubts is like a human body without any antibodies in it. People who blithely go through life too busy or indifferent to ask hard questions about why they believe as they do will find themselves defenseless against either the experience of tragedy or the probing questions of a smart skeptic. A person’s faith can collapse almost overnight if she has failed over the years to listen patiently to her own doubts, which should only be discarded after long reflection. ¶ Believers should acknowledge and wrestle with doubts — not only their own but their friends’ and neighbors’. It is no longer sufficient to hold beliefs just because you inherited them. Only if you struggle long and hard with objections to your faith will you be able to provide the grounds for your beliefs to skeptics, including yourself, that are plausible rather than ridiculous or offensive. And, just as important for our current situation, such a process will lead you, even after you come to a position of strong faith, to respect and understand those who doubt.
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.