2 Mark now these words. As yet, I am urging it upon you, what straits we are put to that by putting your minds on the stretch, that is, by your praying for us and for yourselves, God may make enlargement, and give us an outlet: lest some man find in His word an occasion of his own perdition, that word which was preached and put in writing only for healing and salvation. “Every man,” says he, “that does sin, does also iniquity.” Lest haply you make a distinction, “Sin is iniquity.”
Lest you say, A sinner I am, but not a doer of iniquity, “Sin is iniquity. And ye know that to this end was He manifested, that He should take away sin; and there is no sin in Him.” And what does it profit us, that He came without sin? “Every one that sins not, abides in Him: and every one that sins, has not seen Him, neither known Him. Little children, let no man seduce you. He that does righteousness is righteous, even as He is righteous.” This we have already said, that the word “as” is wont to be used of a certain resemblance, not of equality.
“He that does sin is of the devil, because the devil sins from the beginning.” This too we have already said, that the devil created no man, nor begot any, but his imitators are, as it were, born of him. “To this end was the Son of God manifested, that He should undo the works of the devil.” Consequently, to undo (or loose) sins, He that has no sin. And then follows: “Every one that is born of God does not commit sin; for his seed remains in him: and he cannot sin, because he is born of God:” he has drawn the cord tight!— Belike, it is in regard of some one sin that he has said, “Does not sin,” not in regard of all sin: that in this that he says, “Whoso is born of God, does not sin,” you may understand some one particular sin, which that man who is born of God cannot commit: and such is that sin that, if one commit it, it confirms the rest.
What is this sin? To do contrary to the commandment. What is the commandment? “A new commandment give I unto you, that you love one another.” Mark well! This commandment of Christ is called, “love.” By this love sins are loosed. If this love be not kept, the not holding it is at once a grievous sin, and the root of all sins.
Source: Homilies on the First Epistle of John (New Advent)