You, however, are controlled not by the sinful nature but by the Spirit, if the Spirit of God lives in you. And if anyone does not have the Spirit of Christ, he does not belong to Christ.
This is continuing from the previous verses which have described what it is to be controlled by sin. Paul continues here to describe the christian. We are not controlled, or characterised by the sinful nature, we are not always under its power, not constantly rebelling against God. That's what sin is, it's rebellion against God. As Christians we are no longer slaves to sin, but rather slaves the God - willing slaves, thankful slaves for his burden is easy and light, he is good, he is gracious and merciful and perfect. It is a blessing to serve God. But not only that, the Spirit lives in us - God lives in us. How is that possible when we were sinners, rebelling against God and his enemies? Well through Jesus, who is God, who came and died in our place, who took our sin and God's perfect punishment for our sin. Through Jesus who suffered and died in our place so we wouldn't have to suffer eternally. Through jesus who rose from the dead, who conquered sin and death and proved it. Through Jesus with whom we are united, he died for us and rose again and he is preparing a place for us. So how do we tell if we have the spirit in us? Well anyone who genuinely repents and puts their faith in Jesus has the spirit. That's God's promise and he does not lie. (Romans 10:9, Joel 2:32, etc). So how can we tell if other people have the spirit? Well they change from being controlled by sin to being controlled by the spirit - that means a change of life. A change from rebellion to obedience. Genuine Faith in Christ always produces good works - not because those good works save us, but because true faith involves submitting to him, and repentance involves turning away from sin. Therefore, within reason we can discern if someone is a christian by how they act - it isn't definitive, and God alone knows the truth, but it's a very good indicator.
The last point of the verse is about belonging to Christ. It's stated in the negative - the non-christian does not belong to Christ, but that means that the Christian does belong to Christ. What a blessing! We belong to the creator of the universe, we are his people. He is faithful and merciful and gracious, accepting us though we are wretched sinners and washing us clean with great personal cost. If he has done all that, then how much more will he sustain us and preserve us in him! As Christians we have full assurance of salvation. If we are genuinely God's people we will never fall away - not because we are good, but because God is all powerful, faithful, good and keeps his promises. Because he won't let us slide away.
Soli Deo Gloria!