Now here Celsus appears to me to have committed a great error, in refusing to those who are sinners by nature, and also by habit, the possibility of a complete transformation, alleging that they cannot be cured even by punishment. For it clearly appears that all men are inclined to sin by nature, and some not only by nature but by practice, while not all men are incapable of an entire transformation. For there are found in every philosophical sect, and in the word of God, persons who are related to have undergone so great a change that they may be proposed as a model of excellence of life.
Among the names of the heroic age some mention Hercules and Ulysses, among those of later times, Socrates, and of those who have lived very recently, Musonius. Not only against us, then, did Celsus utter the calumny, when he said that “it was manifest to every one that those who were given to sin by nature and habit could not by any means— even by punishments— be completely changed for the better,” but also against the noblest names in philosophy, who have not denied that the recovery of virtue was a possible thing for men.
But although he did not express his meaning with exactness, we shall nevertheless, though giving his words a more favourable construction, convict him of unsound reasoning. For his words were: “Those who are inclined to sin by nature and habit, no one could completely reform even by chastisement;” and his words, as we understood them, we refuted to the best of our ability.
Source: Against Celsus (New Advent)