12:1 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.  2 Do not be conformed to this world,  but be transformed by the renewal of your mind, that by testing you may discern what is the will of God, what is good and acceptable and perfect.  (Romans 12:1-2 (ESV)
“Be transformed by the renewal of your mind.” A passive imperative is somewhat an oxymoron. But when God says “let there be light” then there is light. The thing here is that we don’t transform ourselves, we are transformed by the gospel. A person could even say transfigured. And as so many things in Scripture, there is a mystery to it. In a sense Paul is saying don’t harden yourself to the transformation, don’t resist it, let it take hold.
This is true sanctification that is not an active thing progressed in by works, but a transformation that occurs through the hearing of the gospel and participation in the sacraments. We don’t sanctify ourselves, we are sanctified, and the sanctification continues throughout the Christian life on earth. The fullness of this transformation, this transfiguration only occurs in the consummation of our baptism which is our death.
Be transformed. This is how Paul begins the second part of his letter now that he has laid out the gospel. He now gives a series of admonitions and examples of what it will look like to lead a Christian life, but being a Christian cannot be reduced to living in accordance with rules and customs. What follows are the consequences of the gospel, of a life lived in the gospel, of a life that constantly returns to the gospel. It is not the condition of being a Christian, but the consequence of being a Christian, and it is going to look differently for everyone depending on where they are in life, what stations and vocations they hold, and with their own personal battles. Some of this also changes according to societal norms, even as it has its effect on societal norms. But this is also why church life is so necessary for the Christian life, and why it is really impossible to be a Christian whilst refusing to go to church. Our life as Christians, our sanctification, is in those things that make up the life of the church, the proclamation of the gospel and the sacraments. When we partake of these things our lives become living sacrifices, and a “spiritual worship.” What Paul is comparing to here is the temple worship of the OT, which was a shadow of things to come, first a shadow of the atonement that Christ would make for the forgiveness of all sins. But second a shadow of the only thing left for the Christian to offer, a thank offering for the forgiveness of sins. We live in thanksgiving to the Lord because he has earned and given us our salvation.