7 This, if you remember, we gave you to understand in beginning to read this Epistle, that nothing in it is so commended as charity. Even if it seems to speak of various other things, to this it makes its way back, and whatever it says, it will needs bring all to bear upon charity. Let us see whether it does so here. Mark: “Whosoever is born of God does not commit sin.” We ask, what sin? Because if you understand all sin, it will be contrary to that place, “If we say that we have no sin, we deceive ourselves, and the truth is not in us.” Then let him say what sin; let him teach us; lest haply I may have rashly said that the sin here is the violation of charity, because he said above, “He that hates his brother is in darkness, and walks in darkness, and knows not whither he goes, because the darkness has blinded his eyes.” But perhaps he has said something in what comes afterwards, and has mentioned charity by name? See that this circuit of words has this end, has this issue. “Whosoever is born of God, sins not, because His seed remains in him.” The “seed” of God, i.e. the word of God: whence the apostle says, “I have begotten you through the Gospel. And he cannot sin, because he is born of God.” Let him tell us this, let us see in what we cannot sin. “In this are manifested the children of God and the children of the devil. Whosoever is not righteous is not of God, neither he that loves not his brother.” Aye, now indeed it is manifest of what he speaks: “Neither he that loves not his brother.” Therefore, love alone puts the difference between the children of God and the children of the devil. Let them all sign themselves with the sign of the cross of Christ; let them all respond, Amen; let all sing Alleluia; let all be baptized, let all come to church, let all build the walls of churches: there is no discerning of the children of God from the children of the devil, but only by charity. They that have charity are born of God: they that have it not, are not born of God. A mighty token, a mighty distinction! Have what you will; if this alone you have not, it profits you nothing: other things if you have not, have this, and you have fulfilled the law. “For he that loves another has fulfilled the law,” says the apostle: and, “Charity is the fulfilling of the law.” I take this to be the pearl which the merchant man in the Gospel is described to have been seeking, who “found one pearl, and sold all that he had, and bought it.” This is the pearl of price, Charity, without which whatever you may have, profits you nothing: which if alone you have, it suffices you. Now, with faith you see, then with actual beholding you shall see. For if we love when we see not, how shall we embrace it when we see! But wherein must we exercise ourselves? In brotherly love. You may say to me, I have not seen God: can you say to me, I have not seen man? Love your brother. For if you love your brother whom you see, at the same time you shall see God also; because you shall see Charity itself, and within dwells God.
8. “Whosoever is not righteous is not of God, neither he that loves not his brother.” “For this is the message:” mark how he confirms it: “For this is the message which we heard from the beginning, that we should love one another.” He has made it manifest to us that it is of this he speaks; whoso acts against this commandment, is in that accursed sin, into which those fall who are not born of God. “Not as Cain, who was of that wicked one, and slew his brother. And wherefore slew he him? Because his own works were evil, and his brother's righteous.” Therefore, where envy is, brotherly love cannot be. Mark, my beloved. He that envies, loves not. The sin of the devil is in that man; because the devil through envy cast man down. For he fell, and envied him that stood. He did not wish to cast man down that he himself might stand, but only that he might not fall alone. Hold fast in your mind from this that he has subjoined, that envy cannot exist in charity. You have it openly, when charity was praised, “Charity envies not.” There was no charity in Cain; and had there been no charity in Abel, God would not have accepted his sacrifice. For when they had both offered, the one of the fruits of the earth, the other of the offspring of the flock; what think ye, brethren, that God slighted the fruits of the earth, and loved the offspring of the flock? God had not regard to the hands, but saw in the heart: and whom He saw offer with charity, to his sacrifice He had respect; whom He saw offer with envy, from his sacrifice He turned away His eyes. By the good works, then, of Abel, he means only charity: by the evil works of Cain he means only his hatred of his brother. It was not enough that he hated his brother and envied his good works; because he would not imitate, he would kill. And hence it appeared that he was a child of the devil, and hence also that the other was God's righteous one. Hence then are men discerned, my brethren. Let no man mark the tongue, but the deeds and the heart. If any do not good for his brethren, he shows what he has in him. By temptations are men proved.
9. “Marvel not, brethren, if the world hate us.” Must one often be telling you what “the world” means? Not the heaven, not the earth, nor these visible works which God made; but lovers of the world. By often saying these things, to some I am burdensome: but I am so far from saying it without a cause, that some may be questioned whether I said it, and they cannot answer. Let then, even by thrusting it upon them, something stick fast in the hearts of them that hear. What is “the world”? The world, when put in a bad sense, is, lovers of the world: the world, when the word is used in praise, is heaven and earth, and the works of God that are in them; whence it is said, “And the world was made by Him.” Also, the world is the fullness of the earth, as John himself has said, “Not only for our sins is He the propitiator, but (for the sins) of the whole world:” he means, “of the world,” of all the faithful scattered throughout the whole earth. But the world in a bad sense, is, lovers of the world. They that love the world, cannot love their brother.
10. “If the world hate us: we know”— What do we know?— “that we have passed from death unto life”— How do we know? “Because we love the brethren.” Let none ask man: let each return to his own heart: if he find there brotherly love, let him set his mind at rest, because he is “passed from death unto life.” Already he is on the right hand: let him not regard that at present his glory is hidden: when the Lord shall come, then shall he appear in glory. For he has life in him, but as yet in winter; the root is alive, but the branches, so to say, are dry: within is the substance that has the life in it, within are the leaves of trees, within are the fruits: but they wait for the summer. Well then, “we know that we have passed from death unto life, because we love the brethren. He that loves not, abides in death.” Lest ye should think it a light matter, brethren, to hate, or, not to love, hear what follows: “Every one that hates his brother, is a murderer.” How now, if any made light of hating his brother, will he also in his heart make light of murder? He does not stir his hands to kill a man; yet he is already held by God a murderer; the other lives, and yet this man is already judged as his slayer! “Every one that hates his brother is a murderer: and you know that no murderer has eternal life abiding in him.”
Source: Homilies on the First Epistle of John (New Advent)