Summa contra Gentiles1.8 Chapter VIII. In what relation human reason stands to the truth of faith

It would also seem well to observe that sensible things from which human reason derives the source of its knowledge, retain a certain trace of likeness to God, but so imperfect that it proves altogether inadequate to manifest the substance itself of God. For effects resemble their causes according to their own mode, since like action proceeds from like agent; and yet the effect does not always reach to a perfect likeness to the agent. Accordingly human reason is adapted to the knowledge of the truth of faith, which can be known in the highest degree only by those who see the divine substance, in so far as it is able to put together certain probable arguments in support thereof, which nevertheless are insufficient to enable us to understand the aforesaid truth as though it were demonstrated to us or understood by us in itself. “And vet however weak these arguments may be, it is useful for the human mind to be practised therein, so long as it does not pride itself on having comprehended or demonstrated: since although our view of the sublimest things is limited and weak, it is most pleasant to be able to catch but a glimpse of them, as appears from what has been said.

The authority of Hilary is in agreement with this statement: for he says (De Trin.) while speaking of this same truth: Begin by believing these things, advance and persevere; and though I know thou wilt not arrive, I shall rejoice at thy advance. For he who devoutly follows in pursuit of the infinite, though he never come up with it, will always advance by setting forth. Yet pry not into that secret, and meddle not in the mystery of the birth of the infinite, nor presume to grasp that which is the summit of understanding: but understand that there are things thou canst not grasp.