C.S. Lewis on Sustaining BeliefMere Christianity (New York: Simon and Schuster, 1996, orig. 1944) p.124,125.
I am not asking anyone to accept Christianity if his best reasoning tells him that the weight of the evidence is against it. That is not the point at which Faith comes in. But supposing a man’s reason once decides that the weight of the evidence is for it. I can tell that man what is going to happen to him in the next few weeks. There will come a moment when there is bad news, or he is in trouble, or is living among a lot of other people who do not believe it, and all at once his emotions will rise up and carry out a sort of blitz on his belief… Now Faith, in the sense in which I am here using the word, is the art of holding on to things your reason has once accepted, in spite of your changing moods. For moods will change, whatever view your reason takes. I know that by experience. Now that I am a Christian I do have moods in which the whole thing looks very improbable: but when I was an atheist I had moods in which Christianity looked terribly probable.