1 John 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.
This verse is not speaking of one particular act of sin, because if that were the case, it would be impossible for anyone to be saved. Rather, it is speaking of a way of life of sin. Salvation is not a license to sin, but since our sin nature is not eradicated at salvation, we still have the propensity to sin.
Romans 6:1 What shall we say then? Shall we continue in sin, that grace may abound?
Romans 6:2 God forbid. How shall we, that are dead to sin, live any longer therein?
Romans 6:15 What then? shall we sin, because we are not under the law, but under grace? God forbid.
1 John 1:8 If we say that we have no sin, we deceive ourselves, and the truth is not in us.
1 John 1:9 If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.
1 John 1:10 If we say that we have not sinned, we make him a liar, and his word is not in us.
1 John 2:1 My little children, these things write I unto you, that ye sin not. And if any man sin, we have an advocate with the Father, Jesus Christ the righteous:
1 John 2:2 And he is the propitiation for our sins: and not for ours only, but also for the sins of the whole world.