1 John 3:4-6
4. Whosoever committeth sin transgresseth also the law: for sin is the transgression of the law.
4. Quicunque facit peccatum, etiam iniquitatem facit; et peccatum est iniquitas.
5. And ye know that he was manifested to take away our sins; and in him is no sin.
5. Porro nostis quod ille apparuit ut peccata nostra tolleret; et peccatum in eo non est.
6. Whosoever abideth in him sinneth not: whosoever sinneth hath not seen him, neither known him.
6. Quisquis in eo manet, non peccat; quisquis peccat, non vidit eum, nec novit eum.
4 Whosoever committeth, or doeth, sin. The Apostle has already shown how ungrateful we must be to God, if we make but little account of the honor of adoption, by which he of his own goodwill anticipates us, and if we do not, at least, render him mutual love. He, at the same time, introduced this admonition, that our love ought not to be diminished, because the promised happiness is deferred. But now, as men are wont to indulge themselves more than they ought, in evils, he reproves this perverse indulgence, declaring that all they who sin are wicked and transgressors of the law. For it is probable that there were then those who extenuated their vices by this kind of flattery, "It is no wonder if we sin, because we are men; but there is a great difference between sin and iniquity."
This frivolous excuse the Apostle now dissipates, when he defines sin to be a transgression of the divine law; for his object was to produce hatred and horror as to sin. The word sin seems light to some; but iniquity or transgression of the law cannot appear to be so easily forgiven. But the Apostle does not make sins equal, by charging all with iniquity who sin; but he means simply to teach us, that sin arises from a contempt of God, and that by sinning, the law is violated. Hence this doctrine of John has nothing in common with the delirious paradoxes of the Stoics.
Besides, to sin here, does not mean to offend in some instances; nor is the word sin to be taken for every fault or wrong a man may commit.; but he calls that sin, when men with their whole heart run into evil, nor does he understand that men sin, except those who are given up to sin. For the faithful, who are as yet tempted by the lusts of the flesh, are not to be deemed guilty of iniquity, though they are not pure or free from sin, but as sin does not reign in them, John says that they do not sin, as I shall presently explain more fully.
The import of the passage is, that the perverse life of those who indulge themselves in the liberty of sinning, is hateful to God, and cannot be borne with by him, because it is contrary to his Law. It does not hence follow, nor can it be hence inferred, that the faithful are iniquitous; because they desire to obey God, and abhor their own vices, and that in every instance; and they also form their own life, as much as in them lieth, according to the law. But when there is a deliberate purpose to sin, or a continued course in sin, then the law is transgressed. 
5 And ye know that he was manifested, or, hath appeared. He shews by another argument how much sin and faith differ from one another; for it is the office of Christ to take away sins, and for this end was he sent by the Father; and it is by faith we partake of Christ's virtue. Then he who believes in Christ is necessarily cleansed from his sins. But it is said in John 1:29, that Christ takes away sins, because he atoned for them by the sacrifice of his death, that they may not be imputed to us before God: John means in this place that Christ really, and, so to speak, actually takes away sins, because through him our old man is crucified, and his Spirit, by means of repentance, mortifies the flesh with all its lusts. For the context does not allow us to explain this of the remission of sins; for, as I have said, he thus reasons, "They who cease not to sin, render void the benefits derived from Christ, since he came to destroy the reigning power of sin." This belongs to the sanctification of the Spirit.
And in him is no sin He does not speak of Christ personally, but of his whole body.  Wherever Christ diffuses his efficacious grace, he denies that there is any more room for sin. He, therefore, immediately draws this inference, that they sin not who remain in Christ. For if he dwells in us by faith, he performs his own work, that is, he cleanses us from sins. It hence appears what it is to sin For Christ by his Spirit does not perfectly renew us at once, or in an instant, but he continues our renovation throughout life. It cannot then be but that the faithful are exposed to sin as long as they live in the world; but as far as the kingdom of Christ prevails in them, sin is abolished. In the meantime they are designated according to the prevailing principle, that is, they are said to be righteous and to live righteously, because they sincerely aspire to righteousness.
They are said not to sin, because they consent not to sin, though they labor under the infirmity of the flesh; but, on the contrary, they struggle with groaning, so that they can truly testify with Paul that they do the evil they would not.
He says that the faithful abide in Christ, because we are by faith united to him, and made one with him.
6 Whosoever sinneth hath not seen him. According to his usual manner he added the opposite clause, that we may know that faith in Christ and knowledge of him are vainly pretended, except there be newness of life. For Christ is never dormant where he reigns, but the Spirit renders effectual his power. And it may be rightly said of him, that he puts sin to flight, not otherwise than as the sun drives away darkness by its own brightness. But we are again taught in this place how strong and efficacious is the knowledge of Christ; for it transforms us into his image. So by seeing and knowing we are to understand no other thing than faith.
 To do, or to commit, or to work, or to practice, sin, and to sin, are evidently used in the same sense by the Apostle: and to commit or practice sin, according to what he says in his Gospel, (John 8:34,) is the same with being "the servant of sin." It is hence evident, that in the language of John, to do sin, or to sin, means a prevailing or an habitual course of sinning. We might render the fourth verse thus, -- "Every doer of sin, is also the doer of unrighteousness; for sin is unrighteousness," or iniquity, as Calvin renders it. The word anomia, literally, is lawlessness, but it is never used strictly in this sense either in the Sept or the New Testament. The terms by which it is commonly expressed, are, wickedness, iniquity, transgression, unrighteousness. See verse 7. -- Ed  It is generally taken as referring to Christ personally; he being mentioned here as having no sin, because he is in this respect an example to his people; or, according to some, because he was thereby fitted for the office of taking away our sins; or, because he had no sin of his own to take away. Grotius viewed the present as used here for the past tense, -- "and sin was not in him." See a similar instance in John 15:27 -- Ed.
 It is generally taken as referring to Christ personally; he being mentioned here as having no sin, because he is in this respect an example to his people; or, according to some, because he was thereby fitted for the office of taking away our sins; or, because he had no sin of his own to take away. Grotius viewed the present as used here for the past tense, -- "and sin was not in him." See a similar instance in John 15:27 -- Ed.