Do not present your members to sin as instruments for unrighteousness, but present yourselves to God as those who have been brought from death to life, and your members to God as instruments for righteousness. (ESV)
We love the idea of having free will. That is until we realize that our free will gets us into a world of trouble. A proper understanding of the limitations of our will would show us that the only thing our will does without the Lord's intervention is sin! We are rebels to the core. If left to our own "free will" we will choose the bondage of sin every time.
When we are saved by the grace of God we become new creations in Christ Jesus. Whereas, before salvation we were incapable of choosing anything but sinful behavior, we now have the ability to choose what is right. That freedom is something very new for us. With that freedom comes responsibility.
Paul says that we are not to present our members to sin. This idea of presenting our members implies that it is something that our mind chooses to do. It is an act of our will. No one forces us to engage in sinful behavior as a believer in Jesus Christ. We sin because our will has made a choice and our physical members carry it out.
It is important what we do with our bodies. They can be used to bring glory to God or they can be used as instruments of sin. In encouraging us to present our members to God as instruments for righteousness We have been brought from death to life. In other words, we are no longer slaves to sin. We need to have a freedom mentality. We are now free to walk in righteousness.
What are doing with your "free will?" Are you using your will to choose righteousness? This is possible for you because you belong to the Lord. Remember that you have been brought from death to life for a reason. Bring him glory with your body.