Jamieson, Fausset and Brown on 1 Corinthians 3:18

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].