Philosophers too know that people can change
Here too Celsus seems to me badly mistaken, in not allowing complete change to those naturally inclined to sin and accustomed to doing it, since he thinks that they are not healed even by punishments. For it is plain that all of us human beings are naturally inclined to sin, and some are not only inclined to sin but also accustomed to it; yet not all human beings are incapable of complete change. For in every school of philosophy, and in the divine word as well, there are men recorded to have changed so much that they are held up as a pattern of the best life. Some cite, among the heroes, Heracles and Odysseus; among later men, Socrates; and among those of very recent times, Musonius.
So Celsus spoke falsely, not only by our account, when he said that it is plain to everyone that those naturally inclined to sin and accustomed to it could not be brought by anyone, not even under punishment, to a complete change for the better. He spoke falsely also by the account of those who have practised philosophy nobly and have not despaired that it is possible for human beings to take up virtue. But even if he did not set out precisely what he meant, we shall nonetheless give him a fair hearing, and even so we shall show that he does not speak soundly. For he said: "No one could completely change those who are naturally inclined to sin and accustomed to it, not even by punishing them"; and we have overturned, as best we could, the sense his words plainly give.
Greek
Καὶ ἐν τούτοις δ᾿ ὁ Κέλσος πάνυ μοι ἐσφάλθαι δοκεῖ, μὴ διδοὺς τοῖς ἁμαρτάνειν πεφυκόσι καὶ τοῦτο πράττειν εἰθισμένοις τὴν παντελῆ μεταβολὴν, ὅστις οὐδ᾿ ἀπὸ κολάσεων αὐτοὺς οἴεται θερα πεύεσθαι. σαφῶς γὰρ φαίνεται ὅτι πάντες μὲν ἄνθρωποι πρὸς τὸ ἁμαρτάνειν πεφύκαμεν, ἔνιοι δὲ οὐ μόνον πεφύκασιν ὀλλὰ καὶ εἰθισμένοι εἰσὶν ἁμαρτάνειν· ἀλλ᾿ οὐ πάντες ἄνθρωποι ἀπαράδεκτοί εἰσι τῆς παντελοῦς μεταβολῆς. εἰσὶ γὰρ καὶ κατὰ πᾶσαν φιλοσοφίας αἵρεσιν καὶ κατὰ τὸν θεῖον λόγον οἱ τοσοῦτον μεταβεβληκέναι ἱστορούμενοι, ὥστε αὐτοὺς ἐγκεῖσθαι παράδειγμα τοῦ ἀρίστου βίου. καὶ φέρουσί τινες ἡρώων μὲν τὸν Ἡρακλέα καὶ τὸν Ὀδυσσέα, τῶν δ᾿ ὕστερον τὸν Σωκράτην, τῶν δὲ χθὲς καὶ πρώην γεγονότων τὸν Μουσώνιον. οὐ μόνον οὖν καθ᾿ ἡμᾶς ἐψεύσατο ὁ Κέλσος εἰπὼν παντί που δῆλον εἶναι τοὺς ἁμαρτάνειν πεφυκότας καὶ εἰθισμένους ὑπ᾿ οὐδενὸς ἂν οὐδὲ κολαζομένους πάντῃ ἀχθῆναι πρὸς τὴν εἰς τὸ βέλτιον μεταβολὴν. ἀλλὰ καὶ κατὰ τοὺς γενναίως φιλοσοφήσαντας καὶ μὴ ἀπογνόντας τὴν τῆς ἀρετῆς ἀνάληψιν εἶναι δυνατὸν τοῖς ἀνθρώποις. ἀλλ᾿ εἰ καὶ μὴ μετὰ ἀκριβείας ὅπερ ἐβούλετο παρέστησεν. οὐδὲν ἧττον εὐγνωμόνως αὐτοῦ ἀκούοντες καὶ οὕτως αὐτὸν ἐλέγξομεν οὐχ ὑγιῶς λέγοντα. εἶπε μὲν γάρ· τοὺς πεφυκότας ἁμαρτάνειν καὶ εἰθισμένους οὐδεὶς ἂν οὐδὲ κολάζων πάντῃ μεταβάλοι· καὶ τὸ ἐξακουόμενον ἀπὸ τῆς λέξεως ὡς δυνατὸν ἡμῖν ἀνετρέψαμεν.