Do you believe that once your truly saved that you can live a "sin free" life?
John stated, "If we say that we have no sin, we deceive ourselves, and the truth is not in us. If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness. If we say that we have not sinned, we make Him a liar, and His word is not in us" (I John 1:8-10). There was only one who lived sin free, and that was Jesus Christ.
Later, John explains the difference between children of God and children of Satan. "Whoever commits sin also commits lawlessness, and sin is lawlessness. And you know that He was manifested to take away our sins, and in Him there is no sin. Whoever abides in Him does not sin. Whoever sins has neither seen Him nor known Him. Little children, let no one deceive you. He who practices righteousness is righteous, just as He is righteous. He who sins is of the devil, for the devil has sinned from the beginning. For this purpose the Son of God was manifested, that He might destroy the works of the devil. Whoever has been born of God does not sin, for His seed remains in him; and he cannot sin, because he has been born of God. In this the children of God and the children of the devil are manifest: Whoever does not practice righteousness is not of God, nor is he who does not love his brother" (I John 3:4-10).
This particular passage doesn't translate well into English because Greek has a mode of speech for which there isn't a direct English equivalent. John is not contradicting what he said at the beginning of his letter. What he is talking about is whether a person continues or stays in sin.
Let me illustrate it this way. Does a wicked person commit evil 100% of the time? Even the worse villain you can name will at times do something good or kind once in a while. So why do we call him wicked? Because he doesn't stay in the right. He might stray from the path of wickedness once in a while, but he always heads back to sin.
Does a righteous person remains righteous 100% of the time? John tells us that isn't possible. Even the most righteous man you can name does something wrong once in a while. The Old Testament is filled with examples. We see the great faith of Abraham and yet see that on two occasions he lied. We see that David is a man after God's own heart, and yet committed a number of grave sins. Even Moses blundered on at least three occasions. So what makes the difference between the righteous and the wicked? The difference is that these people didn't stay wicked. They repented of their sins and went back to pursuing righteousness. A righteous man might stray from the path of goodness once in a while, but he always heads back to God.
The mode missing in I John 3 is the expression of continuing and abiding action. You can see the translators try to approximate it by using the word "practices" but it doesn't quite do it justice.