Then when he says, For since, he states the reason why the faithful are saved by the foolishness of preaching. He had already stated that the word of the cross is foolishness to them that perish, but the power of God to them that are saved; for it pleased God by the folly of what we preach, i.e., by the preaching which human wisdom considers foolish, to save them that believe; and this because the world; i.e., worldly men, knew not God by wisdom taken from things of the world; and this in the wisdom of God.
For divine wisdom, when making the world, left indications of itself in the things of the world, as it says in Sirach (1:10): “He poured wisdom out upon all his works,” so that the creatures made by God’s wisdom are related to God’s wisdom, whose signposts they are, as a man’s words are related to his wisdom, which they signify. And just as a disciple reaches an understanding of the teacher’s wisdom by the words he hears from him, so man can teach an understanding of God’s wisdom by examining the creatures He made, as it says in Romans (1:20): “His invisible nature has been clearly perceived in the things that have been made.”
But on account of the vanity of his heart man wandered from the right path of divine knowledge; hence it says in Jn (1:10): “He was in the world, and the world was made through him, yet the world knew him not.” Consequently, God brought believers to a saving knowledge of Himself by other things, which are not found in the natures of creatures; on which account worldly men, who derive their notions solely from human things, considered them foolish: things such as the articles of faith. It is like a teacher who recognizes that his meaning was not understood from the words he employed, and then tried to use other words to indicate what he meant.
This is a brief selection from Aquinas's discussion of I Corinthians 1:17b-25, in his Commentary on the work, as translated by Fabian Larcher, O.P. (see here (PDF) par.50 on p. 17). Aquinas's scriptural commentaries are insufficiently studied; I highly recommend this one to those who have a taste for scriptural commentary. (HT: Redeem the Time)