"Therefore I urge you, brethren, by the mercies of God, to present your bodies a living and holy sacrifice, acceptable to God, which is your spiritual service of worship" (Romans 12:1). I've been studying through Romans recently and I just came to chapter 12. It was tempting to breeze past the first two verses because I've known them for so long. However, when I started to study them, the Holy Spirit showed me some things that made me see the verses in a whole new light. (Funny how that happens!)
Here's what I learned: the mercies of God in context with chapter 11 have to do with Salvation. Our response to the fact that God has saved us should be worship. This worship should express itself in sacrifice. When Paul talked about sacrifice, he was conjuring up the picture of an animal sacrifice. The Jews of old offered burnt offerings and sacrifices not just for sin and atonement, but to show their gratitude to God. (Our sacrifice cannot be for atonement because Christ has done that once for all; therefore, it must be a sacrifice of worship.) Our sacrifice is a living one--our bodies. So, the offering up of one's body to God should not be an act of duty, but a "service of worship" in light of God's vast mercy.