For Divine faith is supernatural both in the principle which elicits the acts and in the objects or truths upon which it falls. The principle which elicits assent to a truth which is beyond the grasp of the human mind must be that same mind illumined by a light superior to the light of reason, viz. the light of faith, and since, even with this light of faith, the intellect remains human, and the truth to be believed remains still obscure, the final assent of the intellect must come from the will assisted by Divine grace, as seen above. But both this Divine light and this Divine grace are pure gifts of God, and are consequently only bestowed at His good pleasure.
It is here that the heroism of faith comes in; our reason will lead us to the door of faith but there it leaves us; and God asks of us that earnest wish to believe for the sake of the reward — "I am thy reward exceeding great" — which will allow us to repress the misgivings of the intellect and say, "I believe, Lord, help Thou my unbelief." As St. Augustine expresses it, "Ubi defecit ratio, ibi est fidei aedificatio" (Sermo ccxlvii, P.L., V, 1157 — "Where reason fails there faith builds up").
from the Cathoic Encyclopedia
