7 Nevertheless David did not act even thus, but found a novel and strange form of moral wisdom: and neither the remembrance of things past, nor the fear of things to come, nor the instigation of the captain, nor the solitude of the place, nor the facility for slaying, nor anything else incited him to kill; but he spared the man who was his enemy, and had given him pain just as if he was some benefactor, and had done him much good. What kind of indulgence then shall we have, if we are mindful of past transgressions, and avenge ourselves on those who have given us pain, whereas that innocent man who had undergone such great sufferings and expected more and worse evils to befall him in consequence of saving his enemy, is seen to spare him, so as to prefer incurring danger himself and to live in fear and trembling, rather than put to a just death the man who would cause him endless troubles?
His moral wisdom then we may perceive, not only from the fact that he did not slay Saul, when there was so strong a compulsion, but also that he did not utter an irreverent word against him, although he who was insulted would not have heard him. Yet we often speak evil of friends when they are absent, he on the contrary not even of the enemy who had done him such great wrong. His moral wisdom then we may perceive from these things: but his lovingkindness and tender care from what he did after these things. For when he had cut off the fringe of Saul's garment, and had taken away the bottle of water he withdrew afar off and stood and shouted, and exhibited these things to him whose life he had preserved, doing so not with a view to display and ostentation, but desiring to convince him by his deeds that he suspected him without a cause as his enemy, and aiming therefore at winning him into friendship. Nevertheless when he had even thus failed to persuade him, and could have laid hands on him, he again chose rather to be an exile from his country and to sojourn in a strange land, and suffer distress every day, in procuring necessary food than to remain at home and vex his adversary. What spirit could be kinder than his? He was indeed justified in saying “Lord remember David and all his meekness.” Let us also imitate him, and let us neither say nor do evil to our enemies, but benefit them according to our power: for we shall do more good to ourselves than to them. “For if you forgive your enemies,” we are told “you shall be forgiven.” Forgive base offenses that you may receive a royal pardon for your offenses; but if any one has done you great wrongs, the greater the wrongs you forgive, the greater will be the pardon which you will receive. Therefore we have been instructed to say “Forgive us, as we forgive,” that we may learn that the measure of our forgiveness takes its beginning in the first place from ourselves. Wherefore in proportion to the severity of the evil which the enemy does to us is the greatness of the benefit which he bestows. Let us then be earnest and eager to be reconciled with those who have vexed us, whether their wrath be just or unjust. For if you are reconciled here, you are delivered from judgment in the other world; but if in the interval while the hatred is still going on, death interrupting steps in and carries the enmity away with it, it follows of necessity that the trial of the case should be brought forward in the other world. As then many men when they have a dispute with one another, if they come to a friendly understanding together outside the law court save themselves loss, and alarm, and many risks, the issue of the case turning out in accordance with the sentiment of each party; but if they severally entrust the affair to the judge the only result to them will be loss of money, and in many cases a penalty, and the permanent endurance of their hatred; even so here if we come to terms during our present life we shall relieve ourselves from all punishment; but if while remaining enemies we depart to that terrible tribunal in the other world we shall certainly pay the utmost penalty at the sentence of the judge there, and shall both of us undergo inexorable punishment: he who is unjustly angry because he is thus unjustly disposed, and he who is justly angry, because he has, however justly, cherished resentment. For even if we have been unjustly ill-treated, we ought to grant pardon to those who have wronged us. And observe how he urges and incites those who have unjustly given pain to reconciliation with those whom they have wronged. “If you offer your gift before the altar, and there rememberest that your brother has ought against you, go your way; first be reconciled to your brother.” He did not say, “assemble, and offer your sacrifice” but “be reconciled and then offer it.” Let it lie there, he says, in order that the necessity of making the offering may constrain him who is justly angry to come to terms even against his will. See how he again prompts us to go to the man who has provoked us when he says “Forgive your debtors in order that your Father may also forgive your trespasses.” For He did not propose a small reward, but one which far exceeds the magnitude of the achievement. Considering all these things then, and counting the recompense which is given in this case and remembering that to wipe away sins does not entail much labour and zeal, let us pardon those who have wronged us. For that which others scarcely accomplish, I mean the blotting out of their own sins by means of fasting and lamentations, and prayers, and sackcloth, and ashes, this it is possible for us easily to effect without sackcloth and ashes and fasting if only we blot out anger from our heart, and with sincerity forgive those who have wronged us. May the God of peace and love, having banished from our soul all wrath and bitterness, and anger, deign to grant that we being closely knit one to another according to the proper adjustment of the parts, may with one accord, one mouth and one soul continually offer up our hymns of thanksgiving due to Him: for to Him be glory and power for ever and ever. Amen.
Source: Homily on the Passage "If your enemy hunger, feed him." (New Advent)