I love this quote from Mere Christianity. I'd add some sort of commentary but really believe it speaks for itself:
Faith seems to be used by Christians in two sense or on two levels....In the first sense it means simply Believe - accepting or regarding as true the doctrines of Christianity. That is fairly simple. But what does puzzle people - at least it used to puzzle me - is the fact that Christians regard faith in this sense as a virtue. I used to ask how on earth it can be a virtue - what is there moral or immoral about believing or not believing a set of statements?...What I did not see then - and a good many people do not see still - was this. I was assuming that if the human mind once accepts a things as true it will automatically go on regarding it as true, until some real reason for reconsidering it turns up. In fact, I was assuming that the human mind is completely ruled by reason. But that is not so....
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 Christianity looks very improbably; but when I was an atheist I had moods in which Christianity looked terribly probable. This rebellion of your moods against your real self is going to come anyway. That is why Faith is such a necessary virtue: unless you teach your moods "where they get off," you can never be either a sound Christian or even a sound atheist, but just a creature dithering to and fro, with its beliefs really dependent on the weather and the state of its digestion. Consequently one must train the habit of Faith.