“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.” —C.S. Lewis
For Lewis, faith was not a matter of trying to force yourself to believe something in spite of the evidence. Rather, it was the ability to not let your moods or emotions affect what you believed to be true. And since our moods and emotions change—and change often—faith is necessary if we don’t want to be constantly waffling in what we believe, changing our mind on whims and feelings.
Faith is not opposed to evidence or reason, so if new evidence arrives, of course, it should be evaluated. But once we reach a belief based on the evidence we see, faith is the ability to hold on to that belief in the midst of the ever changing moods and emotions that we all experience.