The subject of this paragraph begins with the Father and shifts to the Son. At the end, however, the church—its relationship to Christ and among the members—becomes the focus. The major theme of the book of Ephesians is unity. It tells us why we are able to have it and what we must do to maintain it.
Paul describes the church as "a body." This is essential to unity and to preaching the gospel, keeping us from not losing our focus. We have to have God's perspective of what we are. We are a body, meaning a living organism, or by analogy, the human body.
Any organism, like the human body, is unified. Each part cooperating for the good of the whole. Notice that Paul does not use a word like "team." The word "team" has some of the same associations as "body," but it is not as accurate. With "body," Paul not only gets across the concept of association within an organism to accomplish a common work, but it also the sense of a far closer relationship and more critical responsibility, in which each part responds to the will of the head.
We are so close to Jesus Christ that Paul describes us as "His fullness," that is, we fill Him out. We complete Him. Paul does this to relate both the closeness of our association with Christ and our responsibility to Him to do everything in our power to build the strength of both.
The church—we—are Jesus Christ's complement. This is the highest honor a human being can be given! There is nothing greater than to say that we are a part—we fill out, we complete—the body of Jesus Christ! It is as though Jesus Christ our Creator considers Himself incomplete until we become part of Him. He is a Bridegroom, incomplete without His Bride. As a vine, He is incomplete without the branches. As a Shepherd, He is incomplete without His sheep. And so also is He incomplete as a Head without a body, without members, through whom He works and is glorified as they cooperate and yield to Him.
John W. Ritenbaugh
What Is the Work of God Now? (Part 5)