But if we were to reproach those who have been converted with their former lives, then we would have occasion to accuse Phædo also, even after he became a philosopher; since, as the history relates, he was drawn away by Socrates from a house of bad fame to the pursuits of philosophy. Nay, even the licentious life of Polemo, the successor of Xenocrates, will be a subject of reproach to philosophy; whereas even in these instances we ought to regard it as a ground of praise, that reasoning was enabled, by the persuasive power of these men, to convert from the practice of such vices those who had been formerly entangled by them.
Now among the Greeks there was only one Phædo, I know not if there were a second, and one Polemo, who betook themselves to philosophy, after a licentious and most wicked life; while with Jesus there were not only at the time we speak of, the twelve disciples, but many more at all times, who, becoming a band of temperate men, speak in the following terms of their former lives: “For we ourselves also were sometimes foolish, disobedient, deceived, serving various lusts and pleasures, living in malice and envy, hateful, and hating one another.
But after that the kindness and love of God our Saviour towards man appeared, by the washing of regeneration, and renewing of the Holy Ghost, which He shed upon us richly,” we became such as we are. For “God sent forth His Word and healed them, and delivered them from their destructions,” as the prophet taught in the book of Psalms. And in addition to what has been already said, I would add the following: that Chrysippus, in his treatise on the Cure of the Passions, in his endeavours to restrain the passions of the human soul, not pretending to determine what opinions are the true ones, says that according to the principles of the different sects are those to be cured who have been brought under the dominion of the passions, and continues: “And if pleasure be an end, then by it must the passions be healed; and if there be three kinds of chief blessings, still, according to this doctrine, it is in the same way that those are to be freed from their passions who are under their dominion;” whereas the assailants of Christianity do not see in how many persons the passions have been brought under restraint, and the flood of wickedness checked, and savage manners softened, by means of the Gospel. So that it well became those who are ever boasting of their zeal for the public good, to make a public acknowledgement of their thanks to that doctrine which by a new method led men to abandon many vices, and to bear their testimony at least to it, that even though not the truth, it has at all events been productive of benefit to the human race.
Source: Against Celsus (New Advent)