You have said that believers are admonished to obey God’s commands both as evidence of their faith (you cite James) and in a show of gratitude for what we’ve been given. Are there any other verses beside the one in Romans 12 where Paul begs believers to, in light of God’s mercy, present our bodies as a living sacrifice/respond to the Lord out of gratitude?
First, let me correct a misunderstanding. I don’t say that we’re admonished to obey God’s commands as evidence of our faith because I don’t believe that. What I say is because of our faith we will demonstrate good works as a sign of our gratitude for the free gift of salvation. We don’t have to work to make these things happen. By allowing the Holy Spirit to work through us we let them happen. When we’re walking with the Lord we have to override the Holy Spirit’s direction to prevent Him from manifesting good works in our life.
There are numerous places in the New Testament that encourage us to live in submission to the Lord. In addition to Romans 12:2, I think Galatians 4:16-26, Ephes. 4:17-32, Phil 3:16 and Phil. 4:4-9 are good examples among many.