In the Gospel reading which precedes this one, the Lord said: “You have not chosen me, but I have chosen you, and appointed you, that you should go and bring forth fruit, and that your fruit should remain, so that whatever ye shall ask of the Father in my name, He may give it you” (Jn 15:16). ...  But here, in the succeeding lesson which you have heard read, He says: “These things I command you, that you love one another” (Jn 15:17). And by this we are to understand that this is the fruit of ours of which He had said, “I have chosen you that you should go and bring forth fruit, and that your fruit should remain.” What He added--“So that whatever you shall ask of the Father in my name, He may give it you”--He will certainly give us if we love one another. For this is the very thing He gave us who chose us when we had no fruit because we had not chosen him, and appointed us to bear fruit, that is, to love one another, a fruit that we cannot have except in him, just as branches can do nothing apart from the vine (see Jn 15:4-5). Charity, then, is our fruit, which the Apostle explains comes “out of a pure heart and a good conscience and unfeigned faith” (1 Tim 1:5). With this love we love one another, with it we love God. And we would not love one another with a true love if we did not love God. A person loves his neighbor as himself if he loves God, for if he does not love God, he does not love himself. For on these two commandments of love the entire Law and the prophets depend (Mt 22:40). This is our fruit, and to it he was referring when he commanded us, “These things I command you, that you love one another.”

This is why also the Apostle Paul, when he wanted to commend the fruit of the Spirit against the deeds of the flesh, set this principle down: “The fruit of the Spirit is love,” and then, as if they sprang from and were linked with this principle, he wove the others together: “joy, peace, patience, kindness, goodness, faith, meekness, temperance” Gal 5:22). For who truly rejoices who does not love what gives him joy? Who has true peace if he does not have it with one he truly loves? Who is patient and perseveres in the good except by a burning love? Who is kind unless he loves the one he helps? Who is good unless he becomes so by loving? Who is a sound believer without the faith that works by love (Gal 5:6)? Whose meekness is useful if it is not governed by love? Who abstains from what debases except by loving what ennobles? Rightly, then does the Master so often command love, as if only love needs to be commanded, for without it all the other things can be of no benefit and it itself cannot be had without the other things by which a person becomes good. (In Ioannem Tr 87, 1; PL 35, 1852-1853)