A church is its members. Until about five or six years ago I did not understand that. My thinking about church membership was mostly wrong. In reading those words you may assume that I didn't value church membership or was opposed to it for some reason. That's not true. I was pro-church membership, but I still misunderstood it. My experience has been that I was not alone.
Even though I can't think of a better term for it, "church membership" is probably not a very helpful idea in describing what it means to join a local church. This is because of the other ways we think of membership in our culture. Most of the groups we are members of are voluntary organizations that we associate with for our benefit, like clubs or service providers. The local church does not exist to provide a service, it is not a club of kind, like-minded religious people, and, if you are a Christian, your membership is not optional.
A local church is like an outpost or an embassy for Christ's coming Kingdom. The Kingdom is the greater reality and God is the ultimate authority, but in many ways the local church represents God and this coming reality. As citizens of Christ's Kingdom, Christians are called to join these outposts so that they may be identified, connected, and protected. Like embassies, churches declare the interests of the coming Kingdom of God to those around them. This is why the typical ways that we think of membership are not helpful. Citizens of a kingdom are bound to submit to that kingdom's ultimate authority and any authorities ruling as an extension or in place of that ruler. When we see it like this, it is probably most accurate to say that Christians don't join churches; they submit to them.
Submission is not a common idea in the United States during the 21st century, but it is how the Bible thinks about church membership. I have heard many Christians claim that the Bible does not mention church membership. If you are looking for a verse that details how a Christian moves to a new town or is unhappy with their current church, visits several different churches, chooses one, and joins, you would be right. But, if you read the New Testament with an eye for calls for Christians to submit to God, love one another, and preach the Gospel, you will find them everywhere. Church membership is biblical like the revelation of the Trinity and Jesus' claims to be God are biblical, they're not in one place, here or there, because they are everywhere! Every verse about the Church, local churches, Christians, and covenant communities teach us that Christians are called to the privilege of joining and submitting to local churches.
I recognize this may be difficult to read for some. Perhaps you have been wounded by a previous church you were a member of, hurt by leadership, taught that church membership was not biblical, or fear commitment. If any of these describes you, first, I want you to know that I am sorry for any unjust pain local churches have caused you. Unfortunately, even though the sins of Christians are atoned for in Christ, they may still cause hurt. Second, if a church has hurt you, would you consider forgiving them, if you have not already? Finally, as a part of the reconciliation that is possible because of the life, death, and resurrection of Jesus and that is ours when we ask for and grant forgiveness, would you ask God to make you open to what His Word says about the Church, local churches, and His will for Christians to join local churches? If you have been hurt in one of these ways in the past, please know that I am praying for you in a special way in preparation for Sunday morning as we talk about membership.
As I close, I want to include one final note. Being a member of a church is not about ensuring that your name appears on a roster of members (although, I believe you should do that). Joining a church is about submitting to God, the authority of the church, following leadership, loving fellow Christ-followers, carrying burdens, sharing ministry, celebrating the coming Kingdom, and proclaiming the resurrection of Jesus, just to name a few things. This Sunday, we will talk about God's glory in the local church and His call for us to become members of a covenant community. I hope you will make it a priority to be there. The church is its members. It cannot be the same without you.
 Church Membership: How the World Knows Who Represents Jesus by Jonathan Leeman, p. 30.