Let not sin therefore reign in your mortal body, to make you obey its passions. (ESV)
In the first century church one of the biggest challenges was a group of false teachers called the Gnostics. The Gnostics believed that the only thing that mattered was that which was spiritual. If you were saved, your spirit was saved. Your physical body could engage in as much sin as it wanted. As long as you were spiritually saved, you were fine. Paul would have something to say about that.
Notice there is a "therefore" in this verse. That word tells us that what Paul has previously said affects what he is saying now. What has he just been saying? Paul has been expounding on the truth that Christ died to sin for us, and he now lives life to God eternally. We also are to be crucified with Christ, putting our sinful nature to death and walking in newness of life in God.
In light of this truth we are to not let sin reign in our mortal body, making us obey its passions. Sadly, in our world today, we are often led exclusively by our passions. We know this is true in those who do not know Jesus, but tragically many Christians are also led by their passions. They have bought into the lie of the world that we should "follow our hearts."
I am always reminded what Jeremiah said about the heart: "The heart is deceitful above all things, and desperately sick; who can understand it? (Jeremiah 17:9)" If we follow our passions, which is what following our heart is all about, we will be in a world of trouble. The heart deceives us just like Eve was deceived when she followed her heart and ate forbidden fruit.
What are you following? Your heart? That is a disastrous idea. I encourage you to put to death the sin that would reign in your mortal body. Submit yourself, passions included, to the Lordship of Jesus Christ.