In his essay “Religion: Reality or Substitute” C. S. Lewis gives what I think is the best explanation of the nature of faith and its relationship to reason. He asserts that in the New Testament the conflict is never between faith and reason; it is between faith and sight. As it says in Hebrew 11:1, “Now faith is being sure of what we hope for and certain of what we do not see.” It is not reason, but the “look” of things that challenges our faith.
Lewis goes further and suggests most losses of faith have no rational basis at all: “How many of our own sudden temporary losses of faith have a rational basis which would stand examination for a moment?” I had to admit that my faith wavers most when my reason is working least. As Lewis suggests, when bored, jealous, fearful, or lustful I may go looking for reasons to justify letting go of my faith. But it is my mood that is searching for doubts, not my brilliant rationality. I think this may be true for many of us.
Many Christian thinkers have illustrated the ways that reason can come to the aid of faith. The design we see in nature can encourage our faith in a Creator and the testimony of our conscience can point us to a good God who requires righteousness.
Lewis argues, however, that faith can help reason: “Reason may win truths; without Faith she will retain them just so long as Satan pleases. . . .If we wish to be rational, not now and then, but constantly, we must pray for the gift of Faith, for the power to go on believing not in the teeth of reason but in the teeth of lust and terror or jealousy and boredom and indifference that which reason, authority, or experience, or all three, have once delivered to us for truth.” Here Lewis is speaking of faith as a gritty virtue that sustains rationality, not an evanescent devotional emotion.
What we have in our most lucid and morally honest moments come to believe is true, faith gives us the power to keep believing, even in the worst of our moods and during our greatest temptations. Faith helps reason stay true.