Romans 12:14

God makes Christians new people in every respect, so that here too he wants to take away from us the habits of anger which are common to everyone, so that instead of cursing others in anger, which we once did so easily, we might rather …

Paul wants them to exhibit such brotherly love that those who want to persecute them will have no excuse for doing so.

He must not receive a gift from a poor man; so that if he himself has afforded anything, it may be good, inasmuch as it is gratuitous. If any one reviles, he must answer him with a blessing;

That we must not curse. In Exodus: "Thou shalt not curse nor speak ill of the ruler of thy people." Also in the thirty-third Psalm: "Who is the man who desires life, and loveth to see good days? Restrain thy tongue from evil, and thy lips that they speak no guile." Of this same thing in Leviticus: "And the Lord spoke to Moses, saying, Bring forth him who hath cursed abroad outside the camp; and all who heard him shall place their hands upon his head, and all the assembly of the children of Israel shall stone him." Of this same thing in Paul's Epistle to the Ephesians: "Let no evil discourse proceed out of your mouth, but that which is good for the edification of faith, that it may give grace to the hearers." Of this same thing to the Romans: "Blessing, and not cursing." Of this same thing in the Gospel according to Matthew: "He who shall say to his brother, Thou fool! shall be liable to the Gehenna of fire." Of this same matter, according to the same Matthew: "But I say unto you, That every idle word which men shall speak, they shall give account for it in the day of judgment. For by thy words thou shalt be justified, and by thy words thou shalt be condemned."

"Bless them which persecute you; bless, and curse not." After teaching them how they ought to be minded towards one another, and after joining the members closely into one, he next proceeds to lead them forth to the battle without, which he makes easier as from this point. For as he who hath not managed things well with those of his own side, will find more difficulty in arranging affairs with strangers, so he, that has practised himself duly among these, will with the more ease have the advantage of those without also. Hence then Paul also as he goes on in his journey, after the one places the other, and says, "Bless them that persecute you." He did not say, be not spiteful or revengeful, but required something far better. For that a man that was wise might do, but this is quite an angel's part. And after saying "bless," he proceeds, "and curse not," lest we should do both the one and the other, and not the former only. For they that persecute us are purveyors of a reward to us. But if thou art sober-minded, there will be another reward after that one, which thou wilt gain thyself. For he will yield thee that for persecution, but thou wilt yield thyself the one from the blessing of another, in that thou bringest forth a very great sign of love to Christ. For as he that curseth his persecutor, showeth that he is not much pleased at suffering this for Christ, thus he that blesseth showeth the greatness of his love. Do not then abuse him, that thou thyself mayest gain the greater reward, and mayest teach him that the thing is matter of inclination, not of necessity, of holiday and feast, not of calamity or dejection. For this cause Christ Himself said, "Rejoice when men speak all manner of evil against you falsely." Hence too it was that the Apostles returned with joy not from having been evil spoken of only, but also at having been scourged. […]