2 Let us see then what it is to believe in Christ; what to believe that Jesus, He is the Christ. He proceeds: “Whosoever believes that Jesus is the Christ, is born of God.” But what is it to believe that? “And every one that loves Him that begot Him, loves Him also that is begotten of Him.” To faith he has straightway joined love, because faith without love is nothing worth. With love, the faith of a Christian; without love, the faith of a devil: but those who believe not, are worse than devils, more stupid than devils. Some man will not believe in Christ: so far, he is not even upon a par with devils. A person does now believe in Christ, but hates Christ: he has the confession of faith in the fear of punishment, not in love of the crown: thus the devils also feared to be punished. Add to this faith love, that it may become a faith such as the Apostle Paul speaks of, a “faith which works by love:” you have found a Christian, found a citizen of Jerusalem, found a fellow-citizen of the angels, found a pilgrim sighing in the way: join yourself to him, he is your fellow-traveller, run with him, if indeed you also art this. “Every one that loves Him that begot Him, loves Him also that is begotten of Him.” Who “begot”? The Father. Who “is begotten”? The Son. What says he then? “Every one that loves the Father, loves the Son.”
3. “In this we know that we love the sons of God.” What is this, brethren? Just now he was speaking of the Son of God, not of sons of God: lo, here one Christ was set before us to contemplate, and we were told, “Whosoever believes that Jesus is the Christ is born of God: and every one that loves Him that begot,” i.e. the Father, “loves Him also that is begotten of Him,” i.e. the Son, our Lord Jesus Christ. And he goes on: “In this we know that we love the sons of God;” as if he had been about to say, “In this we know that we love the Son of God.” He has said, “the sons of God,” whereas he was speaking just before of the Son of God— because the sons of God are the Body of the Only Son of God, and when He is the Head, we the members, it is one Son of God. Therefore, he that loves the sons of God, loves the Son of God, and he that loves the Son of God, loves the Father; nor can any love the Father except he love the Son, and he that loves the sons, loves also the Son of God. What sons of God? The members of the Son of God. And by loving he becomes himself a member, and comes through love to be in the frame of the body of Christ, so there shall be one Christ, loving Himself. For when the members love one another, the body loves itself. “And whether one member suffer, all the members suffer with it; or one member be honored, all the members rejoice with it.” And then he goes on to say, “Now you are the body of Christ, and members.” John was speaking just before of brotherly love, and said, “He that loves not his brother whom he sees, how can he love God whom he sees not?” But if you love your brother, haply you love your brother and lovest not Christ? How should that be, when you love members of Christ? When therefore you love members of Christ, you love Christ; when you love Christ, you love the Son of God; when you love the Son of God, you love also the Father. The love therefore cannot be separated into parts. Choose what you will love; the rest follow you. Suppose you say, I love God alone, God the Father. You are lying: if you love, you love Him not alone; but if you love the Father, you love also the Son. Behold, do you say, I love the Father, and I love the Son: but this only, the Father God and the Son God, our Lord Jesus Christ, who ascended into heaven, and sits at the right hand of the Father, that Word by which all things were made, and “the Word was made flesh, and dwelt in us:” this alone I love. You are lying; for if you love the Head, you love also the members; but if you love not the members, neither do you love the Head. Do you not quake at the voice uttered by the Head from Heaven on behalf of His members, “Saul, Saul, why do you persecute ME?” The persecutor of His members He called His persecutor: His lover, the lover of His members. Now what are His members, you know, brethren: none other than the Church of God. “In this we know that we love the sons of God, in that we love God.” And how? Are not the sons of God one thing, God Himself another? But he that loves God, loves His precepts. And what are the precepts of God? “A new commandment give I unto you, that you love one another.” Let none excuse himself by another love, for another love; so and so only is it with this love: as the love itself is compacted in one, so all that hang by it does it make one, and as fire melts them down into one. It is gold: the lump is molten and becomes some one thing. But unless the fervor of charity be applied, of many there can be no melting down into one. “That we love God,” by this “know we that we love the sons of God.”
Source: Homilies on the First Epistle of John (New Advent)