What does 1 Corinthians 12:13 mean?

Paul is building an illustration for the church, all the Christians on earth, from the idea of a human body. Every human body is one thing, one person, made up of lots of different parts—"members"—with a wide variety of functions, sizes, and visibility.

Likewise, the church, known as the body of Christ, is one thing with lots of parts. How is this possible? Paul uses the concept of baptism here to imply a union, or a joining-together for a common faith and purpose. Paul may have literal water baptism in mind here. This is something the early church practiced almost immediately after someone converted to Christianity. It's also possible that water baptism is being used as a picture of what happens when someone comes to Christ and receives God's Spirit for the first time. They are said to be baptized into the Spirit, immersed in God's protective, empowering Spirit. In addition, Paul describes every believer in Jesus as drinking of one Spirit. This pictures the Spirit's coming into Christians and occupying them. Every Christian has the Spirit of God (Romans 8:9).

Because every Christian is immersed in and filled up with God's Spirit—the same Spirit for all believers—we are also connected to each other. This allows the church to become one body made up of diverse parts. These differences include nationality and race, gender, physical and intellectual ability, as well as social status. The church in Corinth was especially diverse in these differences.