[S]omeone will say that it is foolish to believe what is not seen, and that one should not believe in things that he cannot see. I answer by saying that the imperfect nature of our intellect takes away the basis of this difficulty. For if man of himself could in a perfect manner know all things visible and invisible, it would indeed be foolish to believe what he does not see. But our manner of knowing is so weak that no philosopher could perfectly investigate the nature of even one little fly. We even read that a certain philosopher spent thirty years in solitude in order to know the nature of the bee. If, therefore, our intellect is so weak, it is foolish to be willing to believe concerning God only that which man can know by himself alone. And against this is the word of Job: “Behold, God is great, exceeding our knowledge [Job 36:26].”
Catechism of St. Thomas Aquinas, Introduction.