69 In the next place, He goes on to say, “You have heard that it has been said, You shall love your neighbour, and hate your enemy: But I say unto you, Love your enemies, do good to them that hate you, and pray for them which persecute you; that you may be the children of your Father which is in heaven: for He commands His sun to rise on the evil and on the good, and sends rain on the just and on the unjust. For if you love them which love you, what reward will you have? Do not even the publicans the same? And if you salute your brethren only, what do ye more than others? Do not even the Gentiles the very same? Be therefore perfect, even as your Father who is in heaven is perfect.” For without this love, wherewith we are commanded to love even our enemies and persecutors, who can fully carry out those things which are mentioned above? Moreover, the perfection of that mercy, wherewith most of all the soul that is in distress is cared for, cannot be stretched beyond the love of an enemy; and therefore the closing words are: “Be therefore perfect, even as your Father who is in heaven is perfect.” Yet in such a way that God is understood to be perfect as God, and the soul to be perfect as a soul.
70. That there is, however, a certain step [in advance] in the righteousness of the Pharisees, which belongs to the old law, is perceived from this consideration, that many men hate even those by whom they are loved; as, for instance, luxurious children hate their parents for restraining them in their luxury. That man therefore rises a certain step, who loves his neighbour, although as yet he hates his enemy. But in the kingdom of Him who came to fulfil the law, not to destroy it, he will bring benevolence and kindness to perfection, when he has carried it out so far as to love an enemy. For the former stage, although it is something, is yet so little that it may be reached even by the publicans as well. And as to what is said in the law, “You shall hate your enemy,” it is not to be understood as the voice of command addressed to a righteous man, but rather as the voice of permission to a weak man.
71. Here indeed arises a question in no way to be blinked, that to this precept of the Lord, wherein He exhorts us to love our enemies, and to do good to those who hate us, and to pray for those who persecute us, many other parts of Scripture seem to those who consider them less diligently and soberly to stand opposed; for in the prophets there are found many imprecations against enemies, which are thought to be curses: as, for instance, that one, “Let their table become a snare,” and the other things which are said there; and that one, “Let his children be fatherless, and his wife a widow,” and the other statements which are made either before or afterwards in the same Psalm by the prophet, as bearing on the case of Judas. Many other statements are found in all parts of Scripture, which may seem contrary both to this precept of the Lord, and to that apostolic one, where it is said, “Bless; and curse not;” while it is both written of the Lord, that He cursed the cities which received not His word; and the above-mentioned apostle thus spoke respecting a certain man, “The Lord will reward him according to his works.”
72. But these difficulties are easily solved, for the prophet predicted by means of imprecation what was about to happen, not as praying for what he wished, but in the spirit of one who saw it beforehand. So also the Lord, so also the apostle; although even in the words of these we do not find what they have wished, but what they have foretold. For when the Lord says, “Woe unto you, Capernaum,” He does not utter anything else than that some evil will happen to her as a punishment of her unbelief; and that this would happen the Lord did not malevolently wish, but saw by means of His divinity. And the apostle does not say, May [the Lord] reward; but, “The Lord will reward him according to his work;” which is the word of one who foretells, not of one uttering an imprecation. Just as also, in regard to that hypocrisy of the Jews of which we have already spoken, whose destruction he saw to be impending, he said, God shall smite you, you whited wall. But the prophets especially are accustomed to predict future events under the figure of one uttering an imprecation, just as they have often foretold those things which were to come under the figure of past time: as is the case, for example, in that passage, “Why have the nations raged, and the peoples imagined vain things?” For he has not said, Why will the heathen rage, and the people imagine vain things? Although he was not mentioning those things as if they were already past, but was looking forward to them as yet to come. Such also is that passage, “They have parted my garments among them, and have cast lots upon my vesture:” for here also he has not said, They will part my garments among them, and will cast lots upon my vesture. And yet no one finds fault with these words, except the man who does not perceive that variety of figures in speaking in no degree lessens the truth of facts, and adds very much to the impressions on our minds.
Source: Our Lord's Sermon on the Mount (New Advent)