And they devoted themselves to the apostles' teaching and the fellowship, to the breaking of bread and the prayers...And day by day, attending the temple together and breaking bread in their homes, they received their food with glad and generous hearts, praising God and having favor with all the people. And the Lord added to their number day by day those who were being saved.
Acts 2:42, 46-47
Church membership is biblical; as we read the book of Acts, one of the things we see is not only is God saving people out of sin and death, but He is saving people to a defined group of people, an organized group with definite boundaries. When God saves a person, that sinner desires to unite himself or herself to the people whom Jesus has saved, the flock which He purchased with His own blood (Acts 20).
The church is the body of the Lord Jesus Christ, instituted by Him for gathering and perfecting the saints in grace and holiness (Eph. 4). The visible church is made up of believers and their children.
The church is comprised of people who are being perfected in holiness, but who are not yet perfect! The church is comprised of people who have recognized their need of God's salvation from their sins, who have been saved from their sins, but who nonetheless still struggle with sin.
The church is the family of God, the Kingdom of God, and she exists chiefly for God's glory by proclaiming the salvation of Christ, by worshiping God according to His word, by the saints loving and encouraging one another toward holiness, and by ministering to our community with the gospel of Christ.