In the introduction to The Reason for God Keller challenges both believers and skeptics to re-examine doubt. Usually we associate doubt with skeptics, but Keller shows why both groups need to rethink this topic.
To believers Keller writes,
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 [a divided culture], such a process will lead you, even after you come to a position of strong faith, to respect and understand those who doubt (xvii).
Then he addresses skeptics.
(...) 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" (xvii, emphasis in the original).
If you're interested in reading more from Keller's book check out the The Reason for God website. The site contains a study guide and several audio lectures in which Tim speaks on the topics addressed in the book.