St. Thomas Aquinas, Commentary on Galatians: 3:15-18
Having proved by authority that the Law does not justify and is not necessary for justification, which is through faith, the Apostle then proves the same point with human reasons.
He says therefore: It is clear that up to now I have been speaking according to the authority of Sacred Scripture, which came not by the will of man, but by the Holy Spirit, as is said in 2 Peter (1:21). But now I speak after the manner of man and after the manners which human reason and human custom follow. Here, indeed, we have an argument to show that in discussions bearing on faith, we may use any truth of any science: “If thou seest in the number of the captives a beautiful woman and lovest her and wilt have her to wife, thou shalt bring her into thy house,” i.e., if you are pleased with worldly wisdom and science, bring it within your boundaries, “and she shall shave her hair, and pare her nails,” i.e., you shall cut away all erroneous opinions (Deut 21:11). This is why in many places in his epistles the Apostle uses the authority of the Gentiles; for example: “Evil communications corrupt good manners” (1 Cor 15:33), and “The Cretans are always liars, evil beasts, slothful bellies” (Tit. 1: 12).