Diving into this lesson at v. 12 of Romans 6 puts one midstream into a powerful current of Paul's theological reflection.
Indeed, v. 12 begins with the word "Therefore" because what follows in v. 12 (and in vv. 12-14) builds off what Paul has already written in vv. 1-11 of Romans 6, namely, that by the grace of God the Christians have mysteriously participated in the death of Christ, so that they (we) are dead to sin and "alive to God in Christ Jesus."
Now, however, Paul recognizes that the power of sin is still active in the world of the believers, so that Christians are confronted with the real peril of falling back into the submission to sin that will result in the Christians' following the baser instincts of human existence rather than God's will and leadership. In one sense Paul is telling the Romans that God has already rescued the Christians out of the mud and cleaned them off through the bath of baptism, so that Paul admonishes these same believers to stay out of the mud, to resist any subsequent urge to return to the mud-bath from which they have already been rescued.
More importantly, however, than the real need to resist returning to the mud is the positive dimension of Paul's teaching--that having been "brought from death to life," the Christians have the opportunity to give themselves to God and to live according to God's righteousness, i.e., God's will, God's power, God's action in the world for salvation, God's purposes.
Verse 14 forms both an admonition and a promise. Here Paul tells his readers that they are truly freed from the power of sin and that they are secure in their freedom because God's grace is the source of their liberty and security. The contrast in this verse between "law" and "grace" brings up a point long debated in Christian circles. What Paul is saying, however, is that in one way of living there is freedom--grace is God's power at work liberating the believers from their involvements with sin; whereas in another way of forming life, the attempt to live life rightly through the observance of the law, one is left essentially on one's own without God's liberating power. For as Paul the Christian sees it, the law (holy, just, and good as it is) is impotent to resolve the dilemma humanity faces in relation to sin, while grace is God's power per se at work doing for the Christians what they cannot do for themselves.
Verse 15 follows v. 14 by employing a rhetorical device know as a diatribe. This manner of reflection and argumentation imaginatively presents two orators in a debate or in an exchange with each other. Given the statement in v. 14, the reader of Paul's letter is to imagine that someone raises a question regarding that declaration (the statement in v. 14). Then, in turn, there is an answer given with explanation to the question that is raised. Paul employs this device in several of his letters, especially in Romans. To sketch the imaginary exchange:
Orator One: For sin will have no dominion over you, since you are not under law but under grace.
Orator Two: What then? Should we sin because we are not under law but under grace?
Orator One: By no means! Do you not know. . . (Paul continues with these remarks from Orator One at least through v. 18).
Paul's point is that independence from law and commitment to and dependence upon grace does not mean that the believers will lapse into a lifestyle of sinfulness. In fact, for Paul it means just the opposite. (As one sees in Romans 7) sin uses the law to deceive humanity into believing that life is manageable essentially on one's own--commitment to and observance of the law can be thought to be sufficient for resisting the power of sin. For Paul, however, the experience of grace exposes the inadequacy of human efforts at godliness and shows the power and sufficiency of grace to live obediently to God's righteousness. Simultaneously Paul exposes the real power of the reality of sin--sin leads to death. Yet, Paul declares that obedience (to God's grace) means righteousness.
Verses 17-18 contrast the conditions of being either "slaves of sin" or "slaves of righteousness." In this same vein, the singer-songwriter Bob Dylan summarized the matter in a song entitled, "Gotta Serve Somebody"--part of which goes,
"You're gonna have to serve somebody, yes indeed,
"You're gonna have to serve somebody.
"Well, it may be the devil or it may be the Lord
"But you're gonna have to serve somebody."
In turn, Paul's teaching in v. 19 is not exactly flattering, even though it had been shown to be true. He starts by explaining that his teaching at this point has its limitations because the Romans themselves have real limitations in their capacity of understanding. (This may not be a ploy that one will elect to imitate in the substance or rhetorical patterning of a sermon!) Then, Paul reminds the Romans that their lives had formerly been less than perfect models of piety. Yet, the Romans were at the time of Paul's writing living differently. Thus, Paul can admonish them to give themselves to righteousness for sanctification--i.e., they are to devote themselves to God and God's work in the world so that they may continue to grow in God's grace.
Finally, in vv. 20-23 Paul continues his line of reasoning, offering (v. 20 begins with the explanatory work "for" in Greek) an explanation of sorts that contrasts sin and its result, death, on the one hand, with righteousness and its results, sanctification and eternal life, on the other. Paul reminds the Romans that involvement with sin meant no involvement with righteousness; but now, involvement with righteousness means no involvement with sin. In v. 23 Paul caps the section with one of the most memorable lines in this letter to the Romans.