God chose wisely in selecting a metaphor for the relation and tie between Christ and His church. What is something that every human being will be able to relate to? Many metaphors would not be understandable over centuries of cultural, social, economic and technological change. If He had compared it to an oxen pulling a plow, the parallel would be vague and insignificant for today's generations of city dwellers. If He had compared it to an automobile and its owner, centuries of Bible readers would have been mystified until the early twentieth century, when the car was finally invented.
But here is something that every human being from the beginning of time until now can understand: the body. We all have one. We all have a body and we all have a head. If you were missing either of those, you sure wouldn't be reading this article! We might not understand all the intricacies of how our bodies function, but we at least understand what it's like to have one. And God has set it up so that with this basic, universal knowledge, we can have a direct insight into the relation between us as the church and Christ as our Savior and Lord.
There are two instances in the Bible that most directly state this relation:
"And He put all things in subjection under [Jesus'] feet, and gave Him as head over all things to the church, which is His body, the fulness of Him who fills all in all." Ephesians 1:22-23
"[Jesus] is also head of the body, the church..." Colossians 1:18
So it's as simple as that. Now let's check out some applications.