The short answer is simple: we believe it is the biblical pattern in both the Old and New Testaments. Scripture uses language such as "added to" and "numbered" to describe the growth of the congregation and how records are kept. The Old Covenant church was "numbered" in Exodus 38 and the book of Numbers. After Peter's great Pentecost sermon in Acts 2, we read that the Lord "added to" the initial band of believers. We see specific instances as the church grew where individuals were known by name; for example, in Acts 6 the apostles were aware of which widows were being neglected. They directed the congregation to select the men qualified to serve them, and then specific men are named.
Jesus, the good and true shepherd, knows his sheep by name, and His undershepherds (elders) are called to shepherd God's flock, his church (Acts 20:28–29; Heb 13:17). With Christ as our pattern, we know them by name so that we can care for them. How can we care for people we don't know? Church membership provides the biblically sanctioned means to commit to one another to walk together in a manner worthy of our calling.
God Keeps Track of His People
In the Old Testament God keeps track of those who are His in great detail. We find obvious examples in the genealogies in 1 Chronicles, but that isn't the only place. Genesis records genealogies as a means of God tracking His holy lineage. In Ezra and Nehemiah we see that Israel went to great lengths to keep records of people. The names of the "congregation" show up at the opening of the New Testament in Matthew and Luke.
Discipline Requires a People
One of the three marks of a true church is discipline. How can we as a church discipline effectively if there is no membership? To what and whom are we held accountable? The Old Testament uses the language of being "cut off" from the assembly (Lev. 20). In the New Testament, if there's no membership and a situation gets to the point of excommunication (1 Cor. 5), what is a person excommunicated from? If there's no church roll, this becomes meaningless.
Visible and Invisible Church
We confess that we believe the church is visible and invisible — invisible being those whom only God knows for sure. But throughout the Scriptures, words like congregation, assembly, church, and body normally refer to the visible manifestation of the church where people gather regularly.
Membership is a commitment of the people to each other and it is a commitment of the shepherds to the sheep. May it be seen as a testimony that we are "friends" (3 John 14) and not "strangers" (3 John 5). May being "members of the household of God" (Eph. 2:19) be seen as a blessing and not a burden. — Phillip Kayser, Church Membership: Is It Biblical
