Yesterday's reading gave us a hint to Paul's concern with the church's behavior: they were allowing human divisions to divide the church. In this part of the reading Paul puts the problem in theological terms. When we talk about communion we're talking about the body of Jesus Christ, and when we talk about the church we're also talking about the body of Christ.
When we allow our prejudices or differences to get in the the way of true, loving community, we are tearing apart the body of our Lord. That's a really big deal. That means we need to do whatever we can to put community first. The church isn't just an organization we take part in, it is a divine body of which we are a part. That's a reality to grow into and a calling for all of us.