25 Hence the Apostle, familiar with the narrow assumption of human thought that what it does not know is not truth, says that he does not speak in the language of knowledge, lest his preaching should be in vain. To save himself from being regarded as a preacher of foolishness he adds that the word of the cross is foolishness to them that perish. He knew that the unbelievers held that the only true knowledge was that which formed their own wisdom, and that, since their wisdom was cognisant only of matters which lay within their narrow horizon, the other wisdom, which alone is Divine and perfect, seemed foolishness to them.
Thus their foolishness actually consisted, in that feeble imagination which they mistook for wisdom. Hence it is that the very things which to them that perish are foolishness are the power of God to them that are saved; for these last never use their own inadequate faculties as a measure, but attribute to the Divine activities the omnipotence of heaven. God rejects the wisdom of the wise and the understanding of the prudent in this sense, that just because they recognise their own foolishness, salvation is granted to them that believe.
Unbelievers pronounce the verdict of foolishness on everything that lies beyond their ken, while believers leave to the power and majesty of God the choice of the mysteries wherein salvation is bestowed. There is no foolishness in the things of God; the foolishness lies in that human wisdom which demands of God, as the condition of belief, signs and wisdom. It is the foolishness of the Jews to demand signs; they have a certain knowledge of the Name of God through long acquaintance with the Law, but the offense of the cross repels them.
The foolishness of the Greeks is to demand wisdom; with Gentile folly and the philosophy of men they seek the reason why God was lifted up on the cross. And because, in consideration for the weakness of our mental powers, these things have been hidden in a mystery, this foolishness of Jews and Greeks turns to unbelief; for they denounce, as unworthy of reasonable credence, truths which their mind is inherently incapable of comprehending. But, because the world's wisdom was so foolish—for previously through God's wisdom it knew not God, that is, the splendour of the universe, and the wonderful order which He planned for His handiwork, taught it no reverence for its Creator— God was pleased through the preaching of foolishness to save them that believe, that is, through the faith of the cross to make everlasting life the lot of mortals; that so the self-confidence of human wisdom might be put to shame, and salvation found where men had thought that foolishness dwelt.
For Christ, Who is foolishness to Gentiles, and offense to Jews, is the Power of God and the Wisdom of God; because what seems weak and foolish to human apprehension in the things of God transcends in true wisdom and might the thoughts and the powers of earth.
Source: On the Trinity (New Advent)