a) The light of faith. — An angel understands truths which are beyond man's comprehension; if then a man were called upon to assent to a truth beyond the ken of the human intellect, but within the grasp of the angelic intellect, he would require for the time being something more than his natural light of reason, he would require what we may call "the angelic light". If, now, the same man were called upon to assent to a truth beyond the grasp of both men and angels, he would clearly need a still higher light, and this light we term "the light of faith" — a light, because it enables him to assent to those supernatural truths, and the light of faith because it does not so illumine those truths as to make them no longer obscure, for faith must ever be "the substance of things to be hoped for, the evidence of things that appear not" (Hebrews 11:1).
Hence St. Thomas (De Veritate, xiv, 9, ad 2) says: "Although the Divinely infused light of faith is more powerful than the natural light of reason, nevertheless in our present state we only imperfectly participate in it; and hence it comes to pass that it does not beget in us real vision of those things which it is meant to teach us; such vision belongs to our eternal home, where we shall perfectly participate in that light, where, in fine, in God's light we shall see light' (Ps. xxxv, 10)."
(b) The necessity of such light is evident from what has been said, for faith is essentially an act of assent, and just as assent to a series of deductive or inductive reasonings, or to intuition of first principles, would be impossible without the light of reason, so, too assent to a supernatural truth would be inconceivable without a supernatural strengthening of the natural light "Quid est enim fides nisi credere quod non vides?" (i.e. what is faith but belief in that which thou seest not?) asks St. Augustine; but he also says: "Faith has its eyes by which it in some sort sees that to be true which it does not yet see—and by which, too, it most surely sees that it does not see what it believes" [Ep. ad Consent., ep. cxx 8 (al. ccxxii), P.L., II, 456].
(c) Again, it is evident that this "light of faith" is a supernatural gift and is not the necessary outcome of assent to the motives of credibility. No amount of study will win it, no intellectual conviction as to the credibility of revealed religion nor even of the claims of the Church to be our infallible guide in matters of faith, will produce this light in a man's mind. It is the free gift of God. Hence the Vatican Council (III, iii;) teaches that "faith is a supernatural virtue by which we with the inspiration and assistance of God's grace, believe those things to be true which He has revealed".
from the Catholic Encyclopedia
