I often hear it said that if you join a local church, you haven't just joined a branch of the church, or a part of the church. You are a member of the church. Each local church is the church. At the same time, the church throughout the world is one.
One thought just struck me that makes this clearer.
In the book of Exodus, the people are told how to build a tabernacle, a tent in which God can live. One of the pieces of furniture in that tabernacle is a golden lampstand. It symbolises that God lives amongst his people.
In the book of Revelation, the risen Christ addresses 7 churches across what is now western Turkey. Chapter 1 has a vision of the risen Christ; chapters 2-3 contain a short message to each of the churches; chapters 3 to 22 record the vision God gave John to pass onto those churches.
In the vision of Revelation 1, the risen Christ is depicted as walking amongst seven golden lampstands. "The seven lampstands are the seven churches" (Revelation 1:20). So each of the 7 churches is symbolised by a golden lampstand.
That means that what the tabernacle symbolised in the Old Testament is fulfilled in each and every one of those seven churches. Each individual church is symbolised by a lampstand, the same object that symbolised the totality of God living amongst his people in the Old Testament.
So each church is a church. Is the church. Not merely part of it.