1 Corinthians 3:18

Passing from the difference between the work of one teacher and that of another, which has occupied him since 1 Corinthians 3:5, the Apostle now returns to the subject from which he branched off there (the magnifying of one teacher above another), and proceeds to show (1 Corinthians 3:18) that merely human wisdom is in itself worthless for spiritual purposes, and, therefore, that the possession of it alone is no reason for the exaltation of the teacher who is endowed with it. For the full meaning of the “wisdom” which the Apostle speaks of here, see 1 Corinthians 1:20.

Let him become a fool — i.e., in the sight of the world, in order that he may become “wise” in the sight of God.