Celsus says turning the other cheek came from Plato

After this, let us look also at his next passage, which runs as follows: “They also have a precept of this kind: not to retaliate against anyone who insults them. ‘And if he strikes one cheek,’ he says, ‘offer the other as well’ (Matthew 5:39; Luke 6:29). This too is ancient, and was said very well before them, but they have related it more crudely. For Plato too has Socrates say this in conversation with Crito: ‘Then one must in no way do wrong. – Certainly not. – Then neither must one who is wronged do wrong in return, as the many think, since one must in no way do wrong. – It seems not. – What then? Must one do harm, Crito, or not? – Surely one must not, Socrates. – What then? To do harm in return when one suffers harm, as the many say: is that just or not just? – Not at all. For doing harm to men surely differs in no way from doing wrong. – You speak the truth. – Then one must neither do wrong in return nor do harm to any man, whatever one may suffer at their hands’ (Plato, Crito 49b–c).

“These are Plato’s words, and again he says this: ‘Consider then, you too, very carefully, whether you share this view and agree with it, and let us begin our deliberation from this point: that it is never right either to do wrong, or to do wrong in return, or, when one suffers harm, to defend oneself by doing harm in return. Or do you stand apart and not share this starting point? For so it seemed to me long ago, and so it seems to me still’ (Plato, Crito 49d–e). This, then, was Plato’s view; but it had been held still earlier by divine men. But about these things, and all the others which they corrupt, let what has been said suffice; and whoever wishes to inquire further into any of them will find out.”