From the introduction of The Reason for God by Timothy Keller...
Only if you [believers] 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....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....Every doubt...is based on a leap of faith.
- p. xvii
The only way to doubt Christianity rightly and fairly is to discern the alternate belief under each of your doubts and then to 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.
- p. xviii