28 So we find in the Old Testament all or nearly all the counsels and precepts which Christ introduces with the words "But I say unto you." Against anger it is written, "My eyes troubled because of anger;" and again, "Better is he that conquers his anger, than he that takes a city." Against hard words, "The stroke of a whip makes a wound; but the stroke of the tongue breaks the bones." Against adultery in the heart, "You shall not covet your neighbor's wife." It is not, "You shall not commit adultery;" but, "You shall not covet."
The apostle, in quoting this, says: "I had not known lust, unless the law had said, You shall not covet." Regarding patience in not offering resistance, a man is praised who "gives his cheek to him that smites him, and who is filled full with reproach." Of love to enemies it is said: "If your enemy hungers, feed him; if he thirsts, give him drink." This also is quoted by the apostle. In the Psalm, too, it is said, "I was a peace maker among them that hated peace;" and in many similar passages.
In connection also with our imitating God in refraining from taking revenge, and in loving even the wicked, there is a passage containing a full description of God in this character; for it is written: "To You alone ever belongs great strength, and who can withstand the power of Your arm? For the whole world before You is as a little grain of the balance; yea, as a drop of the morning dew that falls down upon the earth. But You have mercy upon all, for You can do all things, and winkest at the sins of men, because of repentance.
For You love all things that are, and abhorrest nothing which You have made; for never would You have made anything if You had hated it. And how could anything have endured, if it had not been Your will? Or been preserved, if not called by You? But You spare all; for they are Yours, O Lord, You lover of souls. For Your good Spirit is in all things; therefore chasten them little by little that offend, and warnest them by putting them in remembrance wherein they have offended, that learning their wickedness, they may believe in You, O Lord." Christ exhorts us to imitate this long-suffering goodness of God, who makes the sun to rise upon the evil and the good, and sends rain on the just and on the unjust; that we may not be careful to revenge, but may do good to them that hate us, and so may be perfect, even as our Father in heaven is perfect. From another passage in these ancient books we learn that, by not exacting the vengeance due to us, we obtain the remission of our own sins; and that by not forgiving the debts of others, we incur the danger of being refused forgiveness when we pray for the remission of our own debts: "He that revenges shall find vengeance from the Lord, and He will surely keep his sin in remembrance. Forgive your neighbor the hurt that he has done to you; so shall your sins also be forgiven when you pray. One man bears hatred against another, and does he seek pardon of the Lord? He shows no mercy to a man who is like himself; and does he ask forgiveness of his own sins? If he that is but flesh nourishes hatred, and asks for favor from the Lord, who will entreat for the pardon of his sins?"
Source: Reply to Faustus the Manichaean (New Advent)