Tim Keller, in The Reason for God:
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.
For those of you who have skeptics in your life that you love, this could be a helpful thesis to explore with them. If you've interacted with people who say things like, "There can't be just one true religion," or "If a good, loving God existed, why is there so much suffering in the world," or "Science has disproved the Bible," lovingly encourage such people to doubt their doubts.