The principal signs are miracles, fulfilled prophecy, and the miraculous spread of the Church. Although these signs do not conclusively prove Scripture true, they are nevertheless rational evidence that reassures us that Scripture is authoritative. Quite the contrary, the biographers of Augustine, Anselm, and Aquinas have made it clear that for these saints, faith was very much of the heart as well as the head, involving trust, hope, prayer, feeling, and personal commitment, as well as intellectual assent. This will make more sense when we see how faith aids reason . Second, faith preserves reason from error, for it guides us when we seek answers in areas where reason alone would lead us into error.
Faith thus provides an external guide that keeps reason on track.
Furthermore, as C.S. Lewis so ably pointed out in Miracles , faith in God undergirds human reason. Consequently, if we suppose that God does not exist, then we have no epistemological basis upon which to believe that the faculty of reason is a valid indicator of truth. In a sense, we have no reason to trust our reason. Without faith in God, we live in a meaningless, absurd, irrational world in which we have no reason to think that the human faculty of reason is valid. On the other hand, if we are the product of a rational and loving God, who is both the Creator of the cosmos and the author of our reason, then we have a basis upon which to trust in the validity of reason.