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. Some people say, "I don't believe in Christianity because I can't accept the existence of moral absolutes. Everyone should determine moral truth for him-or herself." Is that a statement they can prove to someone who doesn't share it? No, it is a leap of faith, a deep belief that individual rights operate not only in the political sphere but also in the moral. There is no empirical proof for such a position. So the doubt (of moral absolutes) is a leap. 


- The Reason for God, Tim Keller, locations 221-31 

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. The 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 form Christians for theirs - you will discover that your doubts are not as solid as they first appeared. 

- The reason for God, Tim Keller, locations 221 -31