18. seemeth—that is, is, and is regarded by himself and others.
wise in this world—wise in mere worldly wisdom (1Co 1:20).
let him become a fool—by receiving the Gospel in its unworldly simplicity, and so becoming a fool in the world's sight [Alford]. Let him no longer think himself wise, but seek the true wisdom from God, bringing his understanding into captivity to the obedience of faith [Estius].