Like father, like son

4 Everyone who makes a practice of sinning also practices lawlessness; sin is lawlessness. 5 You know that he appeared in order to take away sins, and in him there is no sin. 6 No one who abides in him keeps on sinning; no one who keeps on sinning has either seen him or known him. 7 Little children, let no one deceive you. Whoever practices righteousness is righteous, as he is righteous. 8 Whoever makes a practice of sinning is of the devil, for the devil has been sinning from the beginning. The reason the Son of God appeared was to destroy the works of the devil. 9 No one born of God makes a practice of sinning, for God's seed abides in him, and he cannot keep on sinning because he has been born of God. 10 By this it is evident who are the children of God, and who are the children of the devil: whoever does not practice righteousness is not of God, nor is the one who does not love his brother (1 Jn 3:4-10).

i) 1 Jn 3:9 has often puzzled commentators and theologians. Wesleyan Arminians and other Christians in the holiness tradition use this as a prooftext for sinless perfection. But one difficulty with that interpretation is that because they can't live up to sunlessness, they redefine sunlessness down to their own level. Sinless perfection becomes its antithesis: antinomianism. If a Christian can't sin, then nothing a Christian does is sinful.  

ii) This interpretation is also in tension with 1 Jn 1:7-10.

iii) More recently, some scholars harmonize the passages by appealing to verbal aspect theory. They think the present tense verb denotes continuous action. Persistent or habitual sin, in contrast to occasional sin. A Christian's life isn't "characterized" by sin.  

However, that's extracting too much theology from a grammatical fine-point. I think it's best to interpret John's statement in light of the surrounding context and his theological framework.

iv) John is probably framing the contrast in somewhat hyperbolic terms for emphasis. 

v) I think John is describing what believers and unbelievers are in principle. If you carry that orientation to its logical conclusion. Unbelievers take after the devil while believers take after God. At present, it isn't that cut-and-dried, but believers and unbelievers are animated by opposing tendencies. Even though they haven't arrived at their respective destinations, you can infer the destination by the general direction in which they are traveling. It's just a matter of time. 

vi) And this dovetails with John's inaugurated eschatology, where the future breaks into the present. The "world" is passing away. The "spirit of the Antichrist" is already here, even thought the Antichrist remains future. So even though the division has yet to be finalized, it's latent in the current bias. 

vii) John is shadowboxing with his theological opponents: the heretical schismatics. That's his immediate target. Not just sinners in general, but sinners who have seen the light, only to recoil and retreat into the shadows (cf. Jn 3:19-20). 

viii) Although this is a bad prooftext for perfectionism, it's a good prooftext for perseverance. John uses a blunt seminal metaphor. Christians must take after the God who spiritually fathered them, for his life-principle remains in them.