1 Jn 3:9 Whosoever is born of God doth not commit sin; for his seed remaineth in him: and he cannot sin, because he is born of God.
I have seen some who mistakenly think John is saying that it is impossible for the Christian to sin, but that contradicts what John says in 1 Jn 1:6-10.
When John says cannot sin does not mean impossible to sin. The law may say you cannot drive over 55mph but that does not mean it is impossible for you to drive over 55mph. The present tense verbs indicates John is sayng one who has become a Christian cannot continue to live in sin, live a sinful lifestyle. If one who lives in adultery (or whatever sin) becomes a Christian, then he cannot continue to live in that adultery (or whatever sin) he must get out of it.
Paul's words to the Corinthians:
Know ye not that the unrighteous shall not inherit the kingdom of God? Be not deceived: neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor abusers of themselves with mankind, Nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners, shall inherit the kingdom of God. And such were some of you: but ye are washed, but ye are sanctified, but ye are justified in the name of the Lord Jesus, and by the Spirit of our God.
Paul says "such WERE some of you" Paul is not saying those Corinthians were perfectly sinless but they had quit continuing living in various sins. Same it true to what Paul said the Ephesians in Eph 2 where the Ephesians before becoming Christians "walked according to the course of this world, had their conversation of life in the flesh of this world" but they did not continue that sinful walk/course having become Christians. Where they perfectly sinless? No, but as long as they walk in the light Christ blood cleanses away all sins.
1 Jn 5:18,19 We know that whosoever is born of God sinneth not; but he that is begotten of God keepeth himself, and that wicked one toucheth him not. And we know that we are of God, and the whole world lieth in wickedness.
Similar idea as sinneth not is present tense one cannot become a Christian and continue to live as a sinner, live a sinful lifestyle. Note that one born of God KEEPETH HIMSELF. What does this mean? The Christian has the responsibility to keep himself from living a lifestyle in sin (v21 admonished to keep himself from idols) less he become lost. Jude commands the Christian to keep himself in the love of God Jude 1:21.