3 Mark well, brethren; we have brought forward somewhat in which, to them that have good understanding, the question is solved. But do we only walk in the way with them that run more swiftly? Those that walk more slowly must not be left behind. Let us turn the matter every way, in such words as we can, in order that it may be brought within reach of all. For I suppose, brethren, that every man is concerned for his own soul, who does not come to Church without cause, who does not seek temporal things in the Church, who does not come here to transact secular business; but comes here in order that he may lay hold upon some eternal thing, promised unto him, whereunto he may attain: he must needs consider how he shall walk in the way, lest he be left behind, lest he go back, lest he go astray, lest by halting he do not attain.
Whoever therefore is in earnest, let him be slow, let him be swift, yet let him not leave the way. This then I have said, that in saying, “Whosoever is born of God sins not,” it is probable he meant it of some particular sin: for else 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.” In this way then the question may be solved. There is a certain sin, which he that is born of God cannot commit; a sin, which not being committed, other sins are loosed, and being committed, other sins are confirmed.
What is this sin? To do contrary to the commandment of Christ, contrary to the New Testament. What is the new commandment? “A new commandment give I unto you, that you love one another.” Whoso does contrary to charity and contrary to brotherly love, let him not dare to glory and say that he is born of God: but whoso is in brotherly love, there are certain sins which he cannot commit, and this above all, that he should hate his brother. And how fares it with him concerning his other sins, of which it is said, “If we say that we have no sin, we deceive ourselves, and the truth is not in us?” Let him hear that which shall set his mind at rest from another place of Scripture; “Charity covers a multitude of sins.”
Source: Homilies on the First Epistle of John (New Advent)