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.
10 In this the children of God are manifest, and the children of the devil: whosoever doeth not righteousness is not of God, neither he that loveth not his brother.
John continues the theme here of us who born of god or who have truly accepted Christ as their savior. He does not mean we can not sin but that the sin nature is gone and as we grow as Christians we are able to resist the temptations that Satan throws at us. Because we are truly born of God, He helps us as we grow to resist sinful nature. As we grow the desire to sin grows smaller if we are remaining in God. In verse 10 again he shows us if you are a child of God you are full of His spirit, and if you follow the devil you do not have the righteous of God nor do you have the love of God for your brother that a Christian should have. These scriptures tell us the fruit we bear reveals who we are. People who habitually sin are of the devil. Those who choose not to sin are of God. This does not mean that a Christian might not commit a single sin; it means that is not their way of life.