9 “And in this,” says he, “we do know Him, if we keep His commandments.” What commandments? “He that says, I know Him, and keeps not His commandments, is a liar, and the truth is not in him.” But still you ask, What commandments? “But whoso,” says he, “keeps His word, in him verily is the love of God perfected.” Let us see whether this same commandment be not called love. For we were asking, what commandments, and he says, “But whoso keeps His word, in him verily is the love of God perfected.”
Mark the Gospel, whether this be not the commandment: “A new commandment,” says the Lord, “give I unto you, that you love one another. — In this we know that we are in Him, if in Him we be perfected.” Perfected in love, he calls them: what is perfection of love? To love even enemies, and love them for this end, that they may be brethren. For not a carnal love ought ours to be. To wish a man temporal good, is good; but though that fail, let the soul be safe. Do you wish life to any that is your friend?
You do well. Do you rejoice at the death of your enemy? You do badly. But haply both to your friend the life you wish him is not for his good, and to your enemy the death you rejoice at has been for his good. It is uncertain whether this present life be profitable to any man or unprofitable: but the life which is with God without doubt is profitable. So love your enemies as to wish them to become your brethren; so love your enemies as that they may be called into your fellowship.
For so loved He who, hanging on the cross, said, “Father, forgive them, for they know not what they do.” For he did not say, Father let them live long, me indeed they kill, but let them live. He was casting out from them the death which is for ever and ever, by His most merciful prayer, and by His most surpassing might. Many of them believed, and the shedding of the blood of Christ was forgiven them. At first they shed it while they raged; now they drank it while they believed.
“In this we know that we are in Him, if in Him we be made perfect.” Touching the very perfection of love of enemies, the Lord admonishing, says, “Be therefore perfect, as your Heavenly Father is perfect. He,” therefore, “that says he abides in Him, ought himself also so to walk, even as He walked.” How, brethren? What does he advise us? “He that says he abides in Him,” i.e., in Christ, “ought himself also so to walk even as He walked.” Haply the advice is this, that we should walk on the sea?
That be far from us! It is this then, that we walk in the way of righteousness. In what way? I have already mentioned it. He was fixed upon the cross, and yet was He walking in this very way: this way is the way of charity, “Father, forgive them, for they know not what they do.” If, therefore, you have learned to pray for your enemy, you walk in the way of the Lord.
Source: Homilies on the First Epistle of John (New Advent)