What does Romans 12:5 mean?
Paul is describing how Christians who have been shown mercy by God—which means all saved believers—should live now. What kind of a life makes sense? The prior verse described a human body. It is just one person, but it has many different parts that serve different functions.Now Paul says that we, as the collective group of Christians, are also a kind of body. We are Christ's body on earth, often referred to as "the church." We are countless different parts in many different places with vastly different jobs, but together we are, by analogy, one unified "being," a single entity with what should be a united purpose.
Paul writes something else about us as part of the body of Christ. None of us exists outside of the body. No Christian is an unattached Christian. We are all members of each other, connected to each other with the purpose of serving each other. This is what God intends to do with us on this side of eternity.