21 And that these words are not a vain boast, shall be made manifest to you from things that have already happened. What could be more stupid than the Ninevites? What more devoid of understanding? Yet, nevertheless, these barbarian, foolish people, who had never yet heard any one teaching them wisdom, who had never received such precepts from others, when they heard the prophet saying, “Yet three days, and Nineveh shall be overthrown,” laid aside, within three days, the whole of their evil customs.
The fornicator became chaste; the bold man meek; the grasping and extortionate moderate and kind; the slothful industrious. They did not, indeed, reform one, or two, or three, or four vices by way of remedy, but the whole of their iniquity. But whence does this appear, says some one? From the words of the prophet; for the same who had been their accuser, and who had said, that “the cry of their wickedness has ascended up even to heaven:” himself again bears testimony of an opposite kind, by saying, “God saw that every one departed from their own evil ways.” He does not say, from fornication, or adultery, or theft, but from their “own evil ways.”
And how did they depart? As God knew, not as man judged of the matter. After this are we not ashamed, must we not blush, if it turns out that in three days only the barbarians laid aside all their wickedness, but that we, who have been urged and taught during so many days, have not got the better of one bad habit? These men had, moreover, gone to the extreme of wickedness before; for when you hear it said, “The cry of their wickedness has come up before me;” you can understand nothing else than the excess of their wickedness.
Nevertheless, within three days they were capable of being transformed to a state of complete virtue. For where the fear of God is, there is no need of days, or of an interval of time; as likewise, on the contrary, days are of no service where there is a want of this fear. For just as in the case of rusted implements, he that rubs them only with water, though he spend a long time on them, will not rid them of all that foulness; but he that puts them in a furnace, will make them presently brighter than even those newly fabricated: so too a soul, stained with the rust of sin, if it cleanse itself slightly, and in a negligent way, and be every day repenting, will gain no further advantage.
But if it cast itself into the furnace, as it were, of the fear of God, it will in a very short time purge all away.
Source: Homilies on the Statues (New Advent)