Paul gives his most in-depth treatment of the Church as the body of Christ in 1 Corinthians 12. The key verses are 12 through 13 and 27:
For just as the body is one and has many members, and all the members of the body, though many, are one body, so it is with Christ. For by one Spirit we were all baptized into one body—Jews or Greeks, slaves or free—and all were made to drink of one Spirit. . . . Now you are the body of Christ and individually members of it.
Paul writes in a similar way in his letter to the Romans:
For as in one body we have many members, and all the members do not have the same function, so we, though many, are one body in Christ, and individually members one of another (Rom. 2:4-5).
The other two passages where Paul refers to the church as the body of Christ are Ephesians 1:22-23 and Colossians 1:24:
“[H]e has put all things under his feet and has made him the head over all things for the church, which is his body, the fullness of him who fills all in all.”
“Now I rejoice in my sufferings for your sake, and in my flesh I complete what is lacking in Christ’s afflictions for the sake of his body, that is, the church.”