13 We are commanded to have only one enemy, the devil. With him be thou never reconciled! But with a brother, never be at enmity in your heart. And if there should be any narrowness of soul, let it be only an ephemeral thing, and never last beyond a day's space. For, “let not the sun,” he says, “go down upon your wrath.” For if, before evening, you are reconciled, you will obtain some pardon from God. But if you remain longer at enmity, that enmity is no longer the result of your being suddenly carried away by anger and resentment, but of wickedness, and of a foul spirit, and one which makes a practice of malice!
And this is not the only terrible thing, that you deprive yourself of pardon, but that the right course becomes still more difficult. For when one day is past, the shame becomes greater; and when the second has arrived, it is still further increased; and if it reach a third, and a fourth day, it will add a fifth. Thus the five become ten; the ten, twenty; the twenty an hundred; and thenceforth the wound will become incurable; for as time goes on, the breach becomes wider. But do thou, O man, give way to none of these irrational passions; nor be ashamed, nor blush, nor say within yourself, “A short time ago we called each other such names, and said a vast number of things fit or not fit to be spoken; and shall I now hurry at once to a reconciliation?
Who then will not blame my excessive easiness?” I answer, no one who has sense will blame your easiness; but when you remain implacable, then, all persons will deride you. Then you will give to the devil the advantage of this wide breach. For the enmity becomes then more difficult to be got rid of, not by mere lapse of time, but from the circumstances too that take place in the meanwhile. For as “charity covers a multitude of sins,” so enmity gives a being to sins that do not exist, and all persons henceforth, are deemed worthy of credit who turn accusers; who rejoice in the ills of others, and blaze abroad what is disgraceful in their conduct.
Source: Homilies on the Statues (New Advent)