From our good friend, Lesslie Newbigin in his most excellent book Household of God,

It is surely a fact of inexhaustible significance that what our Lord left behind Him was not a book, nor a creed, nor a system of thought, nor a rule of life, but a visible community. I think that we protestants cannot too often reflect on that fact. He committed the entire work of salvation to that community. It was not that a community gathered round an idea, so that the idea was primary and the community secondary.  It was that a community called together by the deliberate choice of the Lord Himself, and re-creeated in Him, gradually sought – and is seeking – to make explicit who He is and what He as done.

If you are looking for a good book on the nature and purpose of the church, this is it.