What then? Shall we sin, because we are not under Torah but under grace? Banish the thought!  Do you not know that to whom you present yourselves as slaves it is for obedience, you are slaves to whom you obey, either to sin unto death, or obedience unto righteousness?  But thanks to God that you were slaves of sin, but you obeyed from a heart unto the pattern of teaching to which you were entrusted,  and having been freed from sin, you were enslaved to righteousness.
Paul stated in v. 14 that sin will not master the Roman Christians, because they are not under Torah but under grace. This statement move Paul to spell out the implications of being under grace. Before he does this though he argues against a possible misconception. It could be reason by those who are no longer under Torah that this would lead to sinful behavior. But as Paul as already hinted at and will show more fully in chapter 7 Torah in and of itself is impotent to restrain sin. In fact, the arrival of Torah brought with it the multiplication of transgression. So to anyone who may conceive of life under grace as a reason to sin Paul says, 'Banish the thought!'. Also in v. 14 Paul moves from death and resurrection language to slavery language (For sin will not master you [14a]), and he fills this language out in vv. 15-18.
The crucial piece here is that emancipation from sin does not lead to freedom from everything and everyone. Rather freedom from slavery to sin brings the Christian into slavery to righteousness, which is ironically true freedom. Paul emphasizes this by showing there are only two ways to live. One can either be a slave of sin, which leads to death or one can be a slave to obedience which leads to righteousness. These are the only two options because on is a slave to what they obey (16b). Paul, by way of thanksgiving, reminds the Romans that though they were at one time slaves of sin, they are no longer that, for they have obeyed from the heart. This statement it seems to me is important for it reminds us that Paul has already talked about a doing of the Law that is not about circumcision but is from the heart (2.14-15). This type of hear obedience does not come from being under Torah but from being under Grace.
One question I have for you, Paulos, is does Paul view Torah and Grace as powers (cf., Flesh and Spirit in Gal 5) or salvation-historical periods (cf., Torah and Faith/fulness in Gal 2-3), or both?
Application Question: Paulos, it seems to me from how Paul has argued in this section and in Romans 6 in general, that Christians should expect to see a discernable obedience in their lives. Because of the already/not yet there will not be perfection until we see Jesus as he is (1 John). But because we are under Grace, and because we are enslaved to righteousness, and because we are dead to sin and alive to God, we should work hard to be obedient, and any other way of living is contrary to the teaching of this passage. What do you think?