Adam Clarke on 1 Corinthians 1:18

For the preaching of the cross - Ὁ λογος γαρ ὁ του σταυρου, The doctrine of the cross; or the doctrine that is of or concerning the cross; that is, the doctrine that proclaims salvation to a lost world through the crucifixion of Christ.

Is to them that perish foolishness - There are, properly speaking, but two classes of men known where the Gospel is preached: απολλυμενοι, the unbelievers and gainsayers, who are perishing; and σοζομενοι, the obedient believers, who are in a state of salvation. To those who will continue in the first state, the preaching of salvation through the merit of a crucified Savior is folly. To those who believe this doctrine of Christ crucified is the power of God to their salvation; it is divinely efficacious to deliver them from all the power, guilt, and pollution of sin.