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.
Source: Homilies on the First Epistle of John (New Advent)