He realized his lack of knowledge formed the basis of his unbelief. As Lewis explained in a letter written shortly after his transition, “What has been holding me back . . . has not been so much a difficulty in believing as a difficulty in knowing . . . you can’t believe a thing while you are ignorant what the thing is.”

Nicholi, Armand (2002-04-03). The Question of God (p. 93). Simon & Schuster, Inc.. Kindle Edition.

When he made that decision, he began to feel the presence of Him whom he had desperately wanted not to meet. Finally, Lewis surrendered. “You must picture me alone in that room in Magdalen, night after night, feeling, whenever my mind lifted even for a second from my work, the steady, unrelenting approach of Him whom I so earnestly desired not to meet. That which I greatly feared had at last come upon me. In the Trinity term . . . I gave in, and admitted that God was God and knelt and prayed: perhaps, that night the most dejected and reluctant convert in all England.”

Nicholi, Armand (2002-04-03). The Question of God (p. 85). Simon & Schuster, Inc.. Kindle Edition.

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. 


- The reason for God, Tim Keller, locations 200-210

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