"I appeal to you therefore, brothers, by the mercies of God, to present your bodies as a living sacrifice, holy and acceptable to God, which is your spiritual worship."
From chapters 1-11, Paul has been outlining the glorious gospel of grace. He has shown everyone how to have a relationship with God through Christ and be righteous before him. In this section Paul now moves toward the practical applications of God's great grace and mercy. Here is how a Christian should respond to God's grace and mercy given freely in Jesus. Here is how a Christian should live.
The "therefore" that begins this verse points back to all the truth Paul has given in the previous 11 chapters. Christians are to give all they have to God because he has given all he is to us in Christ. Paul uses sacrificial language from the Old Testament to express a Christian's new life in Christ. We are to give their entire selves, their bodies, both body and soul, to the one called Lord Jesus. A living sacrifice connotes someone who was dead and is now alive. Moreover, we will never be put to death again for sin. We will never die at all! Our resurrection is secure in the one who was raised from the dead.
Jesus has also fulfilled all the Old Testament animal sacrifices outlined in Leviticus 1-7. They are no longer necessary. Jesus' death on the cross is now sufficient for all sins to be forever forgiven.
Finally, a Christian's "spiritual worship" is his whole life totally offered freely to God. Jesus offered his life for us. He paid for all of our sin by giving his entire life to us. In response, we offer freely our lives for him.
What else can we do in response to God's amazing grace and mercy revealed in Christ?