10 Show mercy then, as men of merciful hearts; because in loving enemies also, you love brethren. Think not that John has given no precept concerning love of our enemy, because he has not ceased to speak of brotherly love. You love brethren. “How,” do you say, “do we love brethren?” I ask wherefore you love an enemy. Wherefore do you love him? That he may be whole in this life? What if it be not expedient for him? That he may be rich? What if by his very riches he shall be blinded?
That he may marry a wife? What if he shall have a bitter life of it? That he may have children? What if they shall be bad? Uncertain therefore are these things which you seem to wish for your enemy, in that you love him; they are uncertain. Wish for him that he may have with you eternal life; wish for him that he may be your brother: when you love him, you love a brother. For you love in him not what he is, but what you wish that he may be. I once said to you, my beloved, if I mistake not: There is a log of timber lying in sight; a good workman has seen the log, not yet planed, just as it was hewn from the forest, he has taken a liking to it, he would make something out of it.
For indeed he did not love it to this end that it should always remain thus. In his art he has seen what it shall be, not in his liking what it is; and his liking is for the thing he will make of it, not for the thing it is. So God loved us sinners. We say that God loved sinners: for He says, “They that are whole need not the Physician, but they that are sick.” Did He love us sinners to the end we should still remain sinners? As timber from the wood our Carpenter saw us, and had in His thoughts the building He would make thereof, not the unwrought timber that it was.
So too you see your enemy striving against you, raging, biting with words, exasperating with contumelies, harassing with hatred: you have regard to this in him, that he is a man. You see all these things that are against you, that they were done by man; and you see in him that he was made by God. Now that he was made man, was God's doing: but that he hates you, is his doing; that he has ill-will at you, is his doing. And what do you say in your mind? Lord, be merciful to him, forgive him his sins, strike terror into him, change him.
You love not in him what he is, but what you wish him to be. Consequently, when you love an enemy, you love a brother. Wherefore, perfect love is the loving an enemy: which perfect love is in brotherly love. And let no man say that John the apostle has admonished us somewhat less, and the Lord Christ somewhat more. John has admonished us to love the brethren; Christ has admonished us to love even enemies. Mark to what end Christ has bidden you to love your enemies. That they may remain always enemies?
If He bade it for this end, that they should remain enemies, you hate, not lovest. Mark how He Himself loved, i.e. because He would not that they should be still the persecutors they were, He said, “Father, forgive them, for they know not what they do.” Whom He willed to be forgiven, them He willed to be changed: whom He willed to be changed, of enemies He deigned to make brethren, and did in truth make them so. He was killed, was buried, rose again, ascended into heaven: sent the Holy Ghost to His disciples: they began with boldness to preach His name, they did miracles in the name of Him that was crucified and slain: those slayers of the Lord saw them; and they who in rage had shed His blood, by believing drank it.
Source: Homilies on the First Epistle of John (New Advent)