1 Hear intently, I do beseech you, because it is no small matter that we have to cope withal: and I doubt not, because you were intent upon it yesterday, that you have with even greater intentness of purpose come together today. For it is no slight question, how he says in this Epistle, “Whosoever is born of God, sins not,” and how in the same Epistle he has said above, “If we say that we have no sin, we deceive ourselves, and the truth is not in us.” What shall the man do, who is pressed by both sayings out of the same Epistle?
If he shall confess himself a sinner, he fears lest it be said to him, Then are you not born of God; because it is written, “Whosoever is born of God, sins not.” But if he shall say that he is just and that he has no sin, he receives on the other side a blow from the same Epistle, “If we say that we have no sin, we deceive ourselves, and the truth is not in us.” Placed then as he is in the midst, what he can say and what confess, or what profess, he cannot find. To profess himself to be without sin, is full of peril; and not only full of peril, but also full of error: “We deceive ourselves,” says he, “and the truth is not in us, if we say that we have no sin.”
But oh that you had none, and said this! For then would you say truly, and in uttering the truth would have not so much as a vestige of wrong to be afraid of. But, that you do ill if you say so, is because it is a lie that you say, “The truth,” says he, “is not in us, if we say that we have no sin.” He says not, “Have not had;” lest haply it should seem to be spoken of the past life. For the man here has had sins: but from the time that he was born of God, he has begun not to have sins.
If it were so, there would be no question to embarrass us. For we should say, We have been sinners, but now we are justified: we have had sin, but now we have none. He says not this: but what says he? “If we say that we have no sin, we deceive ourselves, and the truth is not in us.” And then after a while he says on the other hand, “Whosoever is born of God sins not.” Was John himself not born of God? If John was not born of God, John, of whom you have heard that he lay in the Lord's bosom; does any man dare engage for himself that in him has taken place that regeneration which it was not granted to that man to have, to whom it was granted to lie in the bosom of the Lord? The man whom the Lord loved more than the rest, him alone had He not begotten of the Spirit?
Source: Homilies on the First Epistle of John (New Advent)