In vv 1-11, Paul had been giving the Romans theological truths about the ramifications of being “immersed” (or “baptized”) into Christ and therefore into His death and life. But up until now he had not given any practical application. Not yet. But armed with the insights about how God regarded them, and how they should reckon themselves, he now presents the application—
Therefore do not let sin reign in your mortal body, that you should obey it in its lusts (Rom 6:12).
“Therefore.” Here is the practical consequence of the truths Paul explained. He mentions four entities in this verse: you, sin, your body, and the body’s lusts.
First, there is you, the believer. You have a responsibility. “Do not let sin reign…” If you’ve been waiting for God to do that for you, you should know it is your responsibility. God commands you to do it. All the truths that Paul has explained in vv 1-11 tell you why you can do it. As Jewett says, “Having been set free from the power of sin by the death and resurrection of Christ, believers have the responsibility of refusing sin’s further domination” (Jewett, Romans, p. 408).
Second, there is sin. You should not continue in sin (v 1), but you might. That is not impossible. You have died to sin (v 2), but you can still insist on disobeying God and reap the deadly consequences of that, even to the point of death (cf. Jas 1:15; 1 John 5:16). You are freed from the domination of sin in your position (v 7), but sin is still there, working to enslave you in your experience (v 6).
Third, there’s your mortal body. For Paul, the body is very important. He writes about it all throughout his epistles (cf. John A. T. Robinson’s The Body: A Study in Pauline Theology). Pauline sanctification is very body-centered. He is constantly mentioning the body in relation to the Christian life, and for good reason—Adam’s fall created enormous trouble for us, including inheriting these subject-to-death (thnētos) bodies. One day, at the Rapture, when the Lord returns for His Body (i.e., the Church), you’ll be transformed and receive a new immortal body at which point the body will be redeemed (1 Cor 15:51-53). But for now, you’re in a mortal body that is sick and dying and has an agenda all its own, because—
Fourth, there are your mortal body’s lusts. Notice that Paul admits the lusts are still there. If you have ever struggled with them, you’re not alone. We all have. But also notice they do not belong to you, but to your body. The born-again part of you does not sin (1 John 3:9), but actually agrees with the law of God (Rom 7:22). But your body is not born again. It is mortal and fallen. The body has perfectly normal and natural desires, too. But those can be twisted into lusts for pleasures that God has forbidden and condemned. A lust is a desire grown to excess. For example, it is one thing to be hungry and another thing to gorge yourself. It is one thing to rest and another to be slothful. It is one thing to find someone attractive and another to fornicate. Sin uses those lusts to assert power over the body. “It is through the lusts or desires of the body that sin is ready to assume control” (Newell, Romans, p. 228). As Peter says, those lusts “wage war against the soul” (1 Pet 2:11). And that inner battle presents a challenge to your life of service to God. “Therefore” you are responsible for something—to not let sin reign in your body.
You—the eternal you that has been reborn through faith in Jesus and which will survive the death of your mortal body—already have the victory over sin, because you’re united to Christ, and He gained the victory. That means you can take up the struggle against sin from the position of victory. Sin wants to rule your body, but sin is a usurper that must be resisted. As Cranfield says, “now they must fight—they must not let sin go on reigning unchallenged over their daily lives, but must revolt in the name of their rightful ruler, God, against sin’s usurping rule” (Cranfield, Romans, p. 138).
“This is the secret of the Christian life,” Eaton explains. “It is first to know the teaching, to know your position as a victor and conqueror already. Then act in faith and joy and put down the sins that still attack you” (Eaton, Living Under Grace, p. 65).