6 Thus then did he encourage the man who has been wronged; but observe also how he unites again the man who has done the wrong to him who has been provoked. First of all by the very manner of the benefit: (for there is no one so degraded and unfeeling as to be unwilling, when he receives meat and drink, to become the servant and friend of him who does this for him): and in the second place through the dread of vengeance. For the passage, “by so doing you shall heap coals of fire on his head” seems indeed to be addressed to the person who gives the food; but it more especially touches him who has caused the annoyance, in order that through fear of this punishment he may be deterred from remaining continually in a state of enmity, and being aware that the reception of food and drink might do him the greatest mischief if he constantly retains his animosity, may suppress his anger. For thus he will be able to quench the coals of fire. Wherefore the proposed punishment and vengeance both induces the one who has been wronged to benefit him who has annoyed him, and it deters and checks him who has given the provocation, and impels him to reconciliation with the man who gives him meat and drink. Paul therefore linked the two persons by a twofold bond, the one depending on a benefit, the other on an act of vengeance. For the difficulty is to make a beginning and to find an opening for the reconciliation: but when that has once been cleared in whatever way it may be, all which follows will be smooth and easy. For even if at first the man who has been annoyed feeds his enemy in the hope of punishing him, yet becoming his friend by the act of giving him food he will be able to expel the desire of vengeance. For when he has become a friend he will no longer feed the man who has been reconciled to him, with an expectation of this kind. Again he who has given the provocation, when he sees the man who has been wronged electing to give him meat and drink, casts out all his animosity, both on account of this deed, and also of his fear of the punishment which is in store for him, even if he be excessively hard and harsh and stony hearted, being put to shame by the benevolence of him who gives him food, and dreading the punishment reserved for him, if he continues to be an enemy after accepting the food.
For this reason Paul did not stop even here in his exhortation, but when he has emptied each side of wrath he proceeds to correct their disposition, saying, “be not overcome of evil.” “For if,” he says, “you continue to bear resentment and to seek revenge you seem indeed to conquer your enemy, but in reality you are being conquered by evil, that is, by wrath: so that if you wish to conquer, be reconciled, and do not make an attack upon your adversary;” for a brilliant victory is that in which by means of good, that is to say by forbearance, you overcome evil, expelling wrath and resentment. But the injured man, when inflamed with passion would not have borne these words. Therefore when he had satisfied his wrath he proceeded to conduct him to the best reason for reconciliation, and did not permit him to remain permanently animated by the wicked hope of vengeance. Do you perceive the wisdom of the lawgiver? And that you may learn that he introduced this law only on account of the weakness of those who would not otherwise be content to make terms among themselves, hear how Christ, when He ordained a law on this same subject did not propose the same reward, as the Apostle; but, having said “Love your enemies, do good to them that hate you,” which means give them food and drink, He did not add “for in so doing you shall heap coals of fire on their heads:” but what did He say? “that you may become like your Father who is in Heaven.” Naturally so, for He was discoursing to Peter, James, and John and the rest of the apostolic band: therefore He proposed that reward. But if you say that even on this understanding the precept is onerous you improve once more the defence which I am making for Paul, but you deprive yourself of every plea of indulgence. For I can prove to you that this which seems to you onerous was accomplished under the Old Dispensation when the manifestation of spiritual wisdom was not so great as it is now. For this reason also Paul did not introduce the law in his own words, but used the very expressions which were employed by him who originally brought it in, that he might leave no room for excuse to those who do not observe it: for the precept “if your enemy hunger feed him, if he thirst give him drink” is not the utterance of Paul in the first instance, but of Solomon. For this reason he quoted the words that he might persuade the hearer that for one who has been advanced to such a high standard of wisdom to regard an old law as onerous and grievous which was often fulfilled by the men of old time, is one of the basest things possible. Which of the ancients, you ask, fulfilled it? There were many, but among others David especially did so more abundantly. He did not indeed merely give food or drink to his enemy, but also rescued him several times from death, when he was in jeopardy; and when he had it in his power to slay him he spared him once, twice, yea many times. As for Saul he hated and abhorred him so much after the countless good services which he had done, after his brilliant triumphs, and the salvation which he had wrought in the matter of Goliath, that he could not bear to mention him by his own name, but called him after his father. For once when a festival was at hand, and Saul, having devised some treachery against him, and contrived a cruel plot, did not see him arrive— “where,” said he, “is the son of Jesse?” He called him by his father's name, both because on account of his hatred he could not endure the recollection of his proper name, and also because he thought to damage the distinguished position of that righteous man by a reference to his low birth—a miserable and despicable thought: for certainly, even if he had some accusation to bring against the father this could in no wise injure David. For each man is answerable for his own deeds, and by these he can be praised and accused. But as it was, not having any evil deed to mention, he brought forward his low birth, expecting by this means to throw his glory into the shade, which in fact was the height of folly. For what kind of offense is it to be the child of insignificant and humble men, “the son of Jesse,” but when David found him sleeping inside the cave, he did not call him the “son of Kish,” but by his title of honour: “for I will not lift up my hand,” he said, “against the Lord's anointed.” So purely free was he from wrath and resentment of injuries: he calls him the Lord's anointed who had done him such great wrongs, who was thirsting for his blood, who after his countless good services had many times attempted to destroy him. For he did not consider how Saul deserved to be treated, but he considered what was becoming for himself both to do and to say, which is the greatest stretch of moral wisdom. How so? When you have got your enemy in a prison, made fast by a twofold, or rather by a triple chain, confinement of space, dearth of assistance, and necessity of sleep, do you not demand a penalty and punishment of him? “No,” he says; “for I am not now regarding what he deserves to suffer, but what it behooves me to do.” He did not look to the facility for slaying, but to the accurate observance of the moral wisdom which was becoming to him. And yet which of the existing circumstances was not sufficient to prompt him to the act of slaughter? Was not the fact that his enemy was delivered bound into his hands a sufficient inducement? For you are aware I suppose that we hasten more eagerly to deeds for which facilities abound, and the hope of success increases our desire to act, which was just what happened then in his case.
Source: Homily on the Passage "If your enemy hunger, feed him." (New Advent)