Membership applies biblical metaphors for the church.

If a local church like the one in Ephesus is a physical body, “members of one another” (Eph. 4:25; 1 Cor. 12:27), then each person is a body part—an arm or a leg, a hand, foot, thumb, or toe. A body part detached from the rest of its body is not beautiful or cool. It’s gruesome. But that’s what it looks like for a Christian to refuse to join a local church—an arm flopping around on the ground. Arms don’t last long when they’re detached from a body. And think of what happens to a church body when you refuse to attach your part, or you violently detach from it . . . well, we can only take the metaphor so far; but Christian, to be what God designed you to be, to be part of the body, you need to figure out which body you’re going to be attached to, and then formally, visibly, join that body—by becoming a member of it. Besides, your church body needs you, even if you’re a tiny pinky toe or an unseen, uncelebrated pancreas.