We acknowledge, however, although Celsus will not have it so, that we do desire to instruct all men in the word of God, so as to give to young men the exhortations which are appropriate to them, and to show to slaves how they may recover freedom of thought, and be ennobled by the word. And those among us who are the ambassadors of Christianity sufficiently declare that they are debtors to Greeks and Barbarians, to wise men and fools, (for they do not deny their obligation to cure the souls even of foolish persons,) in order that as far as possible they may lay aside their ignorance, and endeavour to obtain greater prudence, by listening also to the words of Solomon: “Oh, you fools, be of an understanding heart,” and “Who is the most simple among you, let him turn unto me;” and wisdom exhorts those who are devoid of understanding in the words, “Come, eat of my bread, and drink of the wine which I have mixed for you.
Forsake folly that you may live, and correct understanding in knowledge.” This too would I say (seeing it bears on the point), in answer to the statement of Celsus: Do not philosophers invite young men to their lectures? And do they not encourage young men to exchange a wicked life for a better? And do they not desire slaves to learn philosophy? Must we find fault, then, with philosophers who have exhorted slaves to the practice of virtue? With Pythagoras for having so done with Zamolxis, Zeno with Perseus, and with those who recently encouraged Epictetus to the study of philosophy?
Is it indeed permissible for you, O Greeks, to call youths and slaves and foolish persons to the study of philosophy, but if we do so, we do not act from philanthropic motives in wishing to heal every rational nature with the medicine of reason, and to bring them into fellowship with God, the Creator of all things? These remarks, then, may suffice in answer to what are slanders rather than accusations on the part of Celsus.
Source: Against Celsus (New Advent)