But since Celsus has declared it to be a saying of many Christians, that “the wisdom of this life is a bad thing, but that foolishness is good,” we have to answer that he slanders the Gospel, not giving the words as they actually occur in the writings of Paul, where they run as follow: “If any one among you seems to be wise in this world, let him become a fool, that he may become wise. For the wisdom of this world is foolishness with God.” The apostle, therefore, does not say simply that “wisdom is foolishness with God,” but “the wisdom of this world.”
And again, not, “If any one among you seems to be wise, let him become a fool universally;” but, “let him become a fool in this world, that he may become wise.” We term, then, “the wisdom of this world,” every false system of philosophy, which, according to the Scriptures, is brought to nought; and we call foolishness good, not without restriction, but when a man becomes foolish as to this world. As if we were to say that the Platonist, who believes in the immortality of the soul, and in the doctrine of its metempsychosis, incurs the charge of folly with the Stoics, who discard this opinion; and with the Peripatetics, who babble about the subtleties of Plato; and with the Epicureans, who call it superstition to introduce a providence, and to place a God over all things.
Moreover, that it is in agreement with the spirit of Christianity, of much more importance to give our assent to doctrines upon grounds of reason and wisdom than on that of faith merely, and that it was only in certain circumstances that the latter course was desired by Christianity, in order not to leave men altogether without help, is shown by that genuine disciple of Jesus, Paul, when he says: “For after that, in the wisdom of God, the world by wisdom knew not God, it pleased God by the foolishness of preaching to save them that believe.” Now by these words it is clearly shown that it is by the wisdom of God that God ought to be known.
But as this result did not follow, it pleased God a second time to save them that believe, not by “folly” universally, but by such foolishness as depended on preaching. For the preaching of Jesus Christ as crucified is the “foolishness” of preaching, as Paul also perceived, when he said, “But we preach Christ crucified, to the Jews a stumbling-block, and to the Greeks foolishness; but to them who are called, both Jews and Greeks, Christ the power of God, and wisdom of God.”
Source: Against Celsus (New Advent)