8 For he who comes the first it is, who reaps all the fruit; and when at the entreaty of another you lay aside your anger, the good work is to be accounted his; for you have discharged the law as doing a favour to him, not as obeying God. But if, when no one entreats, when not even the man who has done the injury approaches, or solicits you, you yourself dismissing from your thoughts all shame, and all delay, runnest forward freely to the injurer, and dost quell anger entirely, the good deed becomes wholly your own, and you shall receive all the reward.
If I say, “Practise fasting,” your plea, perchance, is bodily weakness. If I say, “Give to the poor,” it is poverty, and bringing up children. If I say, “Make time for the assembles of the Church,” it is worldly cares. If I say, “Give heed to what is spoken, and consider the power of what is taught,” it is want of learning. If I say, “Correct another,” you say, “When counsel is given him, he takes no heed, for I have often spoken, and been scorned.” Frigid, as such pretences are, yet you have some pretences to allege.
But suppose I say, “Dismiss your anger,” which of these will you then allege? For neither infirmity of body, nor poverty, nor lack of culture, nor want of leisure, nor any other thing of that kind have you to advance; but this sin is above all other the most inexcusable. How will you be able to stretch your hands toward heaven, or how to move your tongue, or to ask pardon? For although God be desirous to pardon your sins, you yourself dost not suffer Him, while you retain that of your fellow-servant!
But suppose that he is cruel, fierce, and savage, and greedy of revenge and retaliation? Why for this reason you ought especially to grant forgiveness. Have you been wronged much, and robbed, and slandered, and injured in matters of the first importance; and do you wish to see your enemy punished? Yet even for this, it will be of use to you to pardon him. For suppose that you yourself takest vengeance, and prosecutest it, either by words, by deeds, or imprecation against the adversary; then God will not afterwards prosecute it too, inasmuch as you have taken your revenge; and not only will He not prosecute the matter for you, but will also demand a penalty of you as a despiser of Himself.
For if this same thing takes place among mankind, viz. that if we beat the servant of another, the master is indignant, and calls the act an insult (for although we be treated injuriously, whether by slaves, or by freemen, it is fitting that we should await the legal decisions of magistrates or masters); if then even among men, to avenge ourselves would not be safe, how much more so when God is the avenger!
Source: Homilies on the Statues (New Advent)