10 But if your concern be, lest your enemy should become worse by your clemency learn this—that it is not thus he is made worse; but far rather if you are unreconciled. For although he were the vilest of men; although he might neither confess nor publish it openly; yet he will silently approve your Christian wisdom, and in his own conscience will respect your gentleness. Should he, however, persist in the same iniquity, while you are endeavouring to soften and conciliate, he will have to abide the heaviest punishment from God.
And that you may know, that although we should pray for our enemies, and for those who have injured us, God does not pardon, if they are likely to become worse by our forbearance, I will mention to you an ancient piece of history. Miriam once spoke against Moses. What then did God do? He sent a leprosy upon her, and made her unclean; notwithstanding that in other respects she had been meek and modest. Afterwards, when Moses himself, the party injured, besought that the wrath might be removed, God consented not: but what did He say?
“If her father had but spit in her face, should she not be ashamed? Let her remain,” says He, “without the camp seven days.” But what He means is to this effect. “If,” says He, “she had a father, and he had put her away from his presence, would she not have undergone the rebuke? I approve you indeed for your fraternal piety, and your meekness and clemency; but I know when is the due time to remit her punishment.” Do thou then show all humanity towards your brother; and do not pardon his offenses in the desire of a greater punishment for him, but of your tenderness and good will; yet understand this very plainly, that the more he shall slight you, while you are labouring to conciliate, so much the greater punishment will he draw down upon himself.
Source: Homilies on the Statues (New Advent)