12 The rest I commend to your thoughts, my beloved, that I may not burden you. For the question we labor to solve is even this— that we call ourselves sinners: for if any man shall say that he is without sin, he is a liar. And in the Epistle of this same John we have found it written, “If we say that we have no sin, we deceive ourselves.” For you should remember what went before: “If we say that we have no sin, we deceive ourselves, and the truth is not in us.” And yet, on the other hand, in what follows you are told, “He that is begotten of God sins not: he that does sin has not seen Him, neither known Him.— Every one that does sin is of the devil:” sin is not of God: this affrights us again.
In what sense are we begotten of God, and in what sense do we confess ourselves sinners? Shall we say, because we are not begotten of God? And what do these Sacraments in regard to infants? What has John said? “He that is begotten of God, sins not.” And yet again the same John has said, “If we say that we have no sin, we deceive ourselves, and the truth is not in us!” A great question it is, and an embarrassing one; and may I have made you intent upon having it solved, my beloved. Tomorrow, in the name of the Lord, what He will give, we will discourse thereof.
Source: Homilies on the First Epistle of John (New Advent)