What does it mean to call the Church the Body of Christ? On the one hand, it has something to do with unity with him: his Holy Spirit is the soul of the Church; the Church is so united that it is as if one person with Jesus. So “mystical body of Christ” indicates some kind of mystical union.
On the other hand, there is St. Paul’s frequent teaching about “many parts, one body.” (This is so ubiquitous in Paul as to be arguably his central teaching, his deepest insight after seeing Jesus as the one he persecuted when he persecuted the Church: most obviously in Romans 12, 1 Corinthians 12-14, Ephesians 1, Ephesians 5, Colossians 1.) In this sense “body” means diversity in unity: “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 of one another. Having gifts that differ according to the grace given to us . . . .”
What is the connection between these senses of “body”? I recently found a great passage about it in Thomas Aquinas’s Commentary on Ephesians 1.
Now a head is related to the other members in three ways: first, by its preeminent placement; second, by the diffusion of powers, since the senses of the other members are derived from their connection to the head; last, by sharing the same nature as the members. . . .
When he says, “which is his body,” he explains what he means, by adding, “and his fullness.” For if someone asks why, in a natural body, there are so many different members, namely head, hands, mouth, etc., the answer is that by their many kinds of actions, they serve the soul, for the soul is the cause, and principle of those actions, and the power of those actions is originally in the soul.
For the body was made for the soul, not the reverse. In this way, the natural body could be called “the fullness of the soul,” for unless there were all the members of the body, the soul could not accomplish all its operations.
It is similar with Christ and the Church. The Church was instituted for Christ, so that the Church is called “his fullness,” the fullness of Christ – that is, so that all the actions which are in Christ’s power could be in a certain sense “filled out” in the members of the Church: when all the spiritual sensitivities, and gifts, and whatever else is in the Church – all of which are first superabundantly in Christ – come forth from him into the members of the Church, and are made perfect in those members.
So Paul adds, “who fills all in all”: namely when the wise one who is a member of the Chruch receives from him the perfect wisdom which is in Christ; the just one receives perfect justice; etc.