Tim Keller had this to say about doubt in The Reason for God:
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 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.
The point is not that doubt is good in and of itself—we ought never be pleased about our doubts (the goal, after all, is to eventually discard them). But rather, the point is that whether a person yet realizes it or not, there are questions that must be thought through and answered, and if we ignore those questions at first, a time will come when they will be forced upon us.
Don’t let those questions build up. Take them slowly, one at a time.
(HT: Jonathan Morrow)