Accordingly, let us pass on to another charge made by Celsus, who is not even acquainted with the words (of our sacred books), but who, from misunderstanding them, has said that “we declare the wisdom that is among men to be foolishness with God;” Paul having said that “the wisdom of the world is foolishness with God.” Celsus says that “the reason of this has been stated long ago.” And the reason he imagines to be, “our desire to win over by means of this saying the ignorant and foolish alone.”
But, as he himself has intimated, he has said the same thing before; and we, to the best of our ability, replied to it. Notwithstanding this, however, he wished to show that this statement was an invention of ours, and borrowed from the Grecian sages, who declare that human wisdom is of one kind, and divine of another. And he quotes the words of Heraclitus, where he says in one passage, that “man's method of action is not regulated by fixed principles, but that of God is;” and in another, that “a foolish man listens to a demon, as a boy does to a man.”
He quotes, moreover, the following from the Apology of Socrates, of which Plato was the author: “For I, O men of Athens, have obtained this name by no other means than by my wisdom. And of what sort is this wisdom? Such, probably, as is human; for in that respect I venture to think that I am in reality wise.” Such are the passages adduced by Celsus. But I shall subjoin also the following from Plato's letter to Hermeas, and Erastus, and Coriscus: “To Erastus and Coriscus I say, although I am an old man, that, in addition to this noble knowledge of 'forms' (which they possess), they need a wisdom, with regard to the class of wicked and unjust persons, which may serve as a protective and repelling force against them.
For they are inexperienced, in consequence of having passed a large portion of their lives with us, who are moderate individuals, and not wicked. I have accordingly said that they need these things, in order that they may not be compelled to neglect the true wisdom, and to apply themselves in a greater degree than is proper to that which is necessary and human.”
Source: Against Celsus (New Advent)