The one who practices sin is of the devil, because the devil has been sinning from the beginning. For this purpose the Son of God was revealed: to destroy the works of the devil.
John now contrasts the one who practices righteousness and is of Christ with the one who does what is sinful and is of the devil. This verse is clearly a parallel construction to verse 7, therefore it intentionally creates a contrast between doing righteous deeds and doing sin, between being righteous like Jesus or being wicked like the devil. This Johannine antithesis draws a stark contrast between two kinds of people: the children of God and the children of the Devil.
“The one who practices sin is of the devil”: Those who are doing the things of Satan must not think that they belong to God because they show by their life that they belong to Satan.
As before, the writer’s use of the present active participle (ὁ ποιῶν τὴν ἁμαρτίαν, “the one who practices sin” [NASB]) indicates a continual state of sin.
Akin, D. L. (2001). 1, 2, 3 John (Vol. 38, p. 146). Nashville: Broadman & Holman Publishers.
In contrast to the one who (literally) “does righteousness” is the man who “does sin” (Haas, de Jonge, and Swellengrebel, 89).918 This present articular participle, once again, should be seen as describing character… Akin interprets the present participle to indicate “a continual state of sin. One’s life is a life of sin…”
Derickson, G. W. (2012). First, Second, and Third John. (H. W. House, W. H. Harris III, & A. W. Pitts, Eds.) (pp. 315–316). Bellingham, WA: Lexham Press.
The idea that John is defining as a “state of sin” points to the unregenerate state wherein someone is under the power and bondage of sin. They are not simply struggling against the powers of darkness and wrestling with sin, but have been subdued by sin’s deception and are living in sin.
Therefore, in this sense, one is either living in sin as a member of Satan’s family or living in righteousness as a member of God’s family. And the end expression of sin in John’s epistle is hate, which reveals those who express their sin in hatred as being children of the Devil:
1 John 2:10–11 (CSB) — 10 The one who loves his brother or sister remains in the light, and there is no cause for stumbling in him. 11 But the one who hates his brother or sister is in the darkness, walks in the darkness, and doesn’t know where he’s going, because the darkness has blinded his eyes.
John 8:44 (CSB) — 44 You are of your father the devil, and you want to carry out your father’s desires. He was a murderer from the beginning and does not stand in the truth, because there is no truth in him. When he tells a lie, he speaks from his own nature, because he is a liar and the father of lies.
“For this purpose the Son of God was revealed: to destroy the works of the devil”: John returns to verse 5 as the foundation for his doctrine by reminding that the mission and nature of Christ is the foundation of our Christian experience. Because Christ came to destroy the works of the Devil, no one who has been saved by Christ will remain in bondage to the works of Satan.