In Chapter 6, Paul anticipates a question raised from the doctrine of justification (i.e., being made right with God), which is, if we are no longer bound to the Mosaic law and we are forgiven thanks to Christ's work then are we free to do as we please i.e., keep on sinning like a recently divorced accountant in Vegas? Paul not only says no but gives a resounding emphatic no that can be properly translated as H-e double hockey sticks No!"
Paul reminds the Roman churches that they are "in Christ", which means that they so identify with Christ that they are to act like him and serve him as their new slave master (or, in better modern parlance) to serve their new king as faithful soldiers. They are to use their bodies as weapons in a war and they can either fight for the side of evil or for the side of justice and godliness. It is our choice as well. There is no middle ground. No room to be wussies in the midst of this spiritual war. No one can be friggin' France! You are either with God (which means you turn your back to a life of persistent, unrepentent sin and sacrifice for others) or you are with the darkness, which means you keep on sinning. Choose sides.