Desiring God Blog recently posted this sketch of a recent sermon: How Important Is Church Membership?
Part of what it means to belong to the body of Christ is to belong to a local body, because God intends the local church to be an expression of his universal church.
The New Testament gives at least five strands of evidence that a definable, local-church membership is necessary:
1. The church is to discipline its members.
2. The reality of excommunication exists.
3. Christians are required to submit to their leaders.
4. Leaders are required to care for their people.
5. The prominent metaphor of the body implies membership.
And this two part series from Pulpit Magazine is helpful on the issue as well.