The Presbyterian Church (USA) has an important task force working on a new "form of government" document. They are guided by the idea that the church has a “missional polity.” The idea is that the church does not have a mission, but that its very essence is mission. The church is part of God's mission to engage the world.
I can almost grasp what it means to say that being is doing -- that the church is what it does. This is akin to my colleagues who talk about "doing theology," a concept (a metaphor, really) that I can almost grasp. I could be convinced to embrace the idea, I think, with more understanding.
But the sociological insight about the church that most of my work has been based on is that the loyalists at the center of the church are loyal to the institution as it actually is. They like the specific congregation they go to, the familiar ways that their presbytery works, and (most of the time) the lumbering steps and counter-steps of the General Assembly. The first mission that the loyalist is committed to is making sure that the actual church is still there tomorrow to do whatever other work it does. For loyalists, the church is an institution with a mission, even if in some larger sense it "is" a mission.