Two interesting points from John Zizioulas' Being as Communion that I want to excerpt.

First is the way he connects the Holy Spirit with Christ as the Body of Christ:

When we now say "Christ" we mean a person and not an individual; we mean a relational entity existing "for me" or "for us."  Here the Holy Spirit is not one who aids us in bridging t he distance between Christ and ourselves, but he is the person of the Trinity who actually realizes in history that which we call Christ, this absolutely relational entity, our Savior.  

I don't think I've ever heard the relationship put quite this way before, and this description helps one to make better sense of the Trinity and particularly of the role of the Holy Spirit.

The second point is his discussion of authentic communion:

Just as a eucharist which is not a transcendance of divisions within a certain locality is a false eucharist, equally a eucharist which takes place in conscious and intentional isolation and separation from other local communities in the world is not a true eucharist.  From that it follows inevitably that a local Church, in order to be not just local but also Church, must be in full communion with the rest of the local Churches in the world.

I like ecumenism, but I've never found denominationalism to be a particular problem, rather it is usually something I like.  This profound critique of it is the most powerful which I've heard.