The quote is attributed to saint Thomas Aquinas. It comes from one of his books. The reference is
S.T. II-II, Q. 1, Art. 5, reply obj. 1:
which says
Unbelievers are in ignorance of things that are of faith, for neither do they see or know them in themselves, nor do they know them to be credible. The faithful, on the other hand, know them, not as by demonstration, but by the light of faith which makes them see that they ought to believe them, as stated above
it has a fairly obvious connection to saint Paul's comment to the Corinthian Christians
The unspiritual man does not receive the gifts of the Spirit of God, for they are folly to him, and he is not able to understand them because they are spiritually discerned. The spiritual man judges all things, but is himself to be judged by no one. "For who has known the mind of the Lord so as to instruct him?" But we have the mind of Christ.
1 Corinthians 2:14-16