4 Everyone who makes a practice of sinning also practices lawlessness; sin is lawlessness. 5 You know that he appeared 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 John 3:4-10 ESV)
Earlier in this letter, John explained that speaking the truth about knowing Jesus Christ means having the actions to back up such a claim. Nobody who merely says they are Christian should be regarded as one if they do not do what Jesus commands.
1 My little children, I am writing these things to you so that you may not sin. But if anyone does sin, we have an advocate with the Father, Jesus Christ the righteous. 2 He is the propitiation for our sins, and not for ours only but also for the sins of the whole world. 3 And by this we know that we have come to know him, if we keep his commandments. 4 Whoever says “I know him” but does not keep his commandments is a liar, and the truth is not in him, 5 but whoever keeps his word, in him truly the love of God is perfected. By this we may know that we are in him: 6 whoever says he abides in him ought to walk in the same way in which he walked.(1 John 2:1-6)
The reason Christians do not sin is that they are God's children, children of light and not of darkness. If they were still in the world of darkness, unbelief and sin, they would have no problem living a sinful, self-righteous, or licentious lifestyle. But now they belong to God and have His seed within them through the work of the Holy Spirit in them. They will not do as the apostates do, who turn from the truth to practice lawlessness. Even if they do sin, they confess it and continue to repent. They cannot sin in the sense of ultimately turning from Christ. He guards them as His own. He set them free from Satan and made them new after the image of God, who loves righteousness.
Obedience is the sign of true and saving faith, not the root of it. It is the fruit that grows out of the tree planted in the soil stained with the Lord's blood. Those who do not have the works to back up their faith, simply do not have faith. But take heart if this worries you, God gives faith as a gift to all whom He chose for salvation before the foundation of the world.
Jesus Christ is the advocate in the sense that He appeals to the Father on behalf of sinners who repent and trust in Him. He has been raised to everlasting life, so His appeals will always be accepted before our God and Father. He is also their propitiation in the sense that His life was given over to death in exchange for sinners who repent and trust in Him. All of their sins are propitiated by His blood, so that they are completely forgiven forever and ever. Whoever does not repent and trust in Him is not forgiven through His blood. Not yet...
But if you turn to Jesus and repent and submit to Him as your Lord and God, doing what He says and not hearing it only, then you have faith and your life will begin to change. And when you commit errors and do not live in perfect obedience to the Lord, your confession of those sins to Jesus, your sadness and willingness to change and obey, will show that you are not sinning in the sense that John described about those who are not genuine. Seek the Lord and do what He says, but remember that He saves by grace as a gift through faith alone. Your works of obedience are simply His power in you causing you to live out your salvation in real life.