If you're a Christian you are a part of God's redemptive plan to unite all things in Christ. That is, you are Christ Church.
Church Membership comes from this simple sentence - and is enforced from what the Bible says in I Corinthians 12.12-13. We are the body of Christ. Many different members. One body. To make known the greatness of God. This happens through the church (Ephesians 3.9-11).
The question for us, then, is: What is my part? How do I contribute - in a way just as important as the preaching pastor - to the building up of the church.