It commits you, the Christian. It’s good to commit yourself to love and serve others. You will grow as a Christian as you commit to encourage, build up, strengthen, serve, rebuke, and pray for other Christians.
Others are committed to you. Becoming a member of a church means joining with an entire group of Christians who have now covenanted to watch over you spiritually. If you’re a member of a church you have an entire church full of people who are publicly committed to loving you and serving you and watching over you.
Elder protection. If you’re a member of a church, that church’s elders and pastors should care for you, pray for you, and personally counsel and teach you. As a member of their church, they are accountable to God for how they lead you (Heb. 13:17).
Safety net. In his book Stop Dating the Church, Josh Harris points out that it’s comforting to know his church would kick him out rather than tolerate his unrepentant sin. Being a church member means that a whole church full of people are committed to helping you live a life that’s pleasing to God, even to the point of excluding you from the church if you stop repenting of sin. While this sounds harsh to some, to those of us who know the deceitfulness of sin, this is an immensely comforting and encouraging reality (Heb. 3:12).
Assurance. Membership is the church’s affirmation of the validity of someone’s profession of faith (Matt. 16:19, 18:18). The church looks at a person’s life, hears their explanation of the gospel and how they came to believe it, and says, “You look like a Christian to us. So join us. Watch over our lives and we’ll watch over yours.” So while membership in a church doesn’t guarantee that someone is a Christian, it should assure believers of the genuineness of their faith.