4 But perchance it will have struck some of you all along, while we have been expounding to you this epistle of blessed John, why it is only “brotherly” love that he so emphatically commends. “He that loves his brother,” says he: and, “a commandment is given us that we love one another.” Again and again it is of brotherly love that he speaks: but the love of God, i.e. the love with which we ought to love God, he has not so constantly named; howbeit, he has not altogether left it unspoken.
But concerning love of an enemy, almost throughout the epistle, he has said nothing. Although he vehemently preaches up and commends charity to us, he does not tell us to love our enemies, but tells us to love our brethren. But just now, when the Gospel was read, we heard, “For if you love them that love you, what reward shall you have? Do not even the publicans this?” How is it then that John the apostle, as the thing of great concern to us in order to a certain perfection, commends brotherly love; whereas the Lord says it is not enough that we love our brethren, but that we ought to extend that love so that we may reach even to enemies?
He that reaches even unto enemies does not overleap the brethren. It must needs, like fire, first seize upon what is nearest, and so extend to what is further off. A brother is nearer to you than any chance person. Again, that person has more hold upon you whom you know not, who yet is not against you, than an enemy who is also against you. Extend your love to them that are nearest, yet do not call this an extending: for it is almost loving yourself, to love them that are close to you. Extend it to the unknown, who have done you no ill. Pass even them: reach on to love your enemies. This at least the Lord commands. Why has the apostle here said nothing about loving an enemy.
Source: Homilies on the First Epistle of John (New Advent)