According to Scripture, why should every Christian join a church?

Every Christian should join a church because Scripture requires it. Granted, there is no direct command in Scripture that says, “Every Christian must join a local church,” but two factors in Scripture indicate that every Christian should be a member of a local church.

1. Jesus established the church to be a public, earthly institution that would mark out, affirm, and oversee those who profess to believe in him (Matt. 16:18-19, 18:15-20). Jesus established the church to publicly declare those who belong to him in order to give the world a display of the good news about himself (John 17:21, 23; see also Eph. 3:10). Jesus wants the world to know who belongs to him and who doesn’t. And how is the world to know who belongs to him and who doesn’t? They are to see which people publicly identify themselves with his people in the visible, public institution he established for this very purpose. They’re to look at the members of his church. And if some people claim to be part of the universal church even though they belong to no local church, they reject Jesus’ plan for them and his church. Jesus intends for his people to be marked out as a visible, public group, which means joining together in local churches.
2. Scripture repeatedly commands Christians to submit to their leaders (Heb. 13:17; 1 Thess. 5:12-13). The only way to do that is by publicly committing to be members of their flock, and saying in effect, “I commit to listening to your teaching, following your direction, and to submitting to your leadership.” There’s no way to obey the scriptural commands to submit to your leaders if you never actually submit to them by joining a local church.