There is a grace within that is transforming me. There is a choice only I can make that leads to my transformation. On the Christan journey both are equally true. In another place Paul says to "work out your own salvation with fear and trembling" because "God is completing a good work in you". On the one hand it appears to be all up to me and on the other hand it appears that God is bringing the changes.
In Romans 12, having already shown us that God is in charge and is forming a people for himself and that God has already accomplished for us what is necessary to be justified, forgiven, and at peace, Paul points to some of our choices and activity. We are to present ourselves as a "living sacrifice". We are not to be conformed but to have our minds renewed. In verse 9 and following, we are to hold fast, love one another, out do one another, not to lag, be ardent, rejoice, persevere, contribute, extend, bless, rejoice, love, and live peaceably with everyone -- these are all actions that require our choice and our participation. God is doing a good work in us but we are partners with God in our own transformation. We make choices, we draw nearer to God, we arrange our lives and work on our attitudes.
12:3 and following is where I will be preaching this Sunday. This passage and similar passages in 1 Corinthians and Ephesians 4 are the root of the Christian understanding of the priesthood of all believers. Every Christian is called to serve and every Christian has been gifted by God to help build up the body of Christ and to be Christ's hands and feet extended in mission and ministry to the world. There are no exceptions. There is no such thing as an "ungifted" Christian. There is no such thing as a Christian who is disengaged from ministry. Everyone is called -- everyone is gifted -- everyone serves. The question we have to ask is not IF we will serve but where can my gifts be best utilized.