From First Corinthians by George T. Montague, SM, commenting on First Corinthians 12:12:
In this section, it is not a matter of some kind of metaphoric “unity in diversity” that could be applied to any group. It is a matter of the kind of unity and diversity that exists in the body of Christ. It is not a question of how the many can be one but how the One, Christ, can be many. This emerges from Paul’s shorthand at the end of this verse. After saying, As a body is one though it has many parts, and all the parts of the body, though many, are one body, he could have said “so also the body of Christ,” meaning the Church. Instead, he says so also Christ, indicating that God’s plan since the resurrection of Jesus is that he be many: the whole Christ, including his members. Here we strike a vein deeply rooted in Paul’s conversion experience, when the risen Lord asked him, “Saul, Saul, why are you persecuting me?” (Acts 9:4). The unity of the Church with Jesus is so intimate that whatever Saul did to the least of Jesus’ brothers, he did to Jesus (Matt 25:40). This is indeed a mystery: as we saw in 6:12–20, Paul thinks of the union of Christians with Christ in realistic and, as it were, physical terms.