Celsus then extracts from the Gospel the precept, “To him who strikes you once, you shall offer yourself to be struck again,” although without giving any passage from the Old Testament which he considers opposed to it. On the one hand, we know that “it was said to them in old time, An eye for an eye, and a tooth for a tooth;” and on the other, we have read, “I say unto you, Whoever shall smite you on the one cheek, turn to him the other also.” But as there is reason to believe that Celsus produces the objections which he has heard from those who wish to make a difference between the God of the Gospel and the God of the law, we must say in reply, that this precept, “Whosoever shall strike you on the one cheek, turn to him the other,” is not unknown in the older Scriptures.
For thus, in the Lamentations of Jeremiah, it is said, “It is good for a man that he bear the yoke in his youth: he sits alone, and keeps silence, because he has borne it upon him. He gives his cheek to him that smites him; he is filled full with reproach.” There is no discrepancy, then, between the God of the Gospel and the God of the law, even when we take literally the precept regarding the blow on the face. So, then, we infer that neither “Jesus nor Moses has taught falsely.” The Father in sending Jesus did not “forget the commands which He had given to Moses:” He did not “change His mind, condemn His own laws, and send by His messenger counter instructions.”
Source: Against Celsus (New Advent)