He imagines, however, that we utter these exhortations for the conversion of sinners, because we are able to gain over no one who is really good and righteous, and therefore open our gates to the most unholy and abandoned of men. But if any one will fairly observe our assemblies we can present a greater number of those who have been converted from not a very wicked life, than of those who have committed the most abominable sins. For naturally those who are conscious to themselves of better things, desire that those promises may be true which are declared by God regarding the reward of the righteous, and thus assent more readily to the statements (of Scripture) than those do who have led very wicked lives, and who are prevented by their very consciousness (of evil) from admitting that they will be punished by the Judge of all with such punishment as befits those who have sinned so greatly, and as would not be inflicted by the Judge of all contrary to right reason. Sometimes, also, when very abandoned men are willing to accept the doctrine of (future) punishment, on account of the hope which is based upon repentance, they are prevented from so doing by their habit of sinning, being constantly dipped, and, as it were, dyed in wickedness, and possessing no longer the power to turn from it easily to a proper life, and one regulated according to right reason. And although Celsus observes this, he nevertheless, I know not why, expresses himself in the following terms: “And yet, indeed, it is manifest to every one that no one by chastisement, much less by merciful treatment, could effect a complete change in those who are sinners both by nature and custom, for to change nature is an exceedingly difficult thing. But they who are without sin are partakers of a better life.”
Source: Against Celsus (New Advent)