There are a number of metaphors for the church in the New Testament: flock (Acts 20:28), household (1 Tim 3:15), temple (1 Cor 3:16), and body (1 Cor 12:12-31), for example. Each image implies that we can identify who belongs and also that survival depends on belonging.
Each of these metaphors contains a clear delineation between inside and out. Sheep belong to a flock, family members to a family, body parts to a body, and these metaphors assume that it’s clear who is and is not a part. There is no confusion about who is wearing the jersey of your team or another.
Neither sheep nor body parts survive on their own; they need to be connected in order to thrive. There is safety in the flock, and there is mutual dependence in the body that benefits each member. Yet these benefits depend on mutual commitment. How do you know to which believers you are to be connected and committed? Church membership seems a good place to start. Be willing to put on the jersey.
“When a person becomes a Christian, he doesn’t just join a local church because it’s a good habit for growing in spiritual maturity. He joins a local church because it’s the expression of what Christ has made him—a member of the body of Christ.”
-Mark Dever, What is a Healthy Church?