Romans 6:13-14

Is not that plain? Paul is not saying the law is done away. He is not saying that grace gives us the right to go out from under the government of God. He really nails this down.

That's a command! Why? Why should not sin have any dominion over us? Because we are under grace (the end of the verse)! Do you see what Paul is saying? Contrary to what these people were saying, that grace does away with law, Paul is saying that the very fact that you are under grace is what nails you to the floor, that you have got to obey the law! You know why? Because grace puts us in a situation of obligation that is so deep that we better obey. Because our own acts, our conduct, has brought upon us the need for grace. The very fact that God has given it obligates us to keep His law.

Romans 6:12-14

Paul makes the same point there as he first begins to discuss sanctification as seen in Romans 12:1: we are to serve God by offering Him our bodies. Sin can control us through our bodies, but it does not need to. So rather than offering our bodies as instruments of sin, we are to offer our bodies to God as instruments for doing His will, that is, for doing good. We do good toward others which glorifies God.