The NT indicates that the church should practice what we call church membership. The church knew who its members were and were not by what they believed and how they lived. While this practice may look more formal or informal from one congregation to the next, it is a practice that exists to one degree or another in every healthy, biblical church. Below are a few strands of evidence from the NT that together make a strong argument for church membership.

Lists: The early churches knew their constituencies with precision. They kept track of who was “added,” sometimes recording the number (Acts 2:41; cf. 2:47; 5:14; 11:24). Within that number, a church could even track its widows and their ages (1 Tim 5:9). 

Specific Ministries and Accountability: Knowing its membership, a church could choose members for specific ministries and hold them accountable. Peter was accountable to the church in Jerusalem for making disciples (Acts 11:2, 18), Barnabas to Antioch (Acts 11:22), and Barnabas and Saul together to Antioch (Acts 13:3; 14:27). Churches sometimes appointed representatives to minister to or inquire of other churches as well (Acts 15:22; 2 Cor 8:19–20; cf. 8:18–21). 

Church Officers: Churches have pastors (1 Tim 3:1–7; Titus 1:5–9), which assumes established bodies of people who are accountable to follow their leadership (cf. 1 Thess 5:12–13; Heb 13:17). Likewise, deacons are men initially suggested by the church from within the church’s membership to its leadership for their respective ministries (Acts 6:3). 

Church Discipline: A defined membership for the church is necessary to exclude those who do not belong (Matt 18:15–17). This exclusion is for those who, in significantly deviating in doctrine or practice, are no longer identifiable as Christians (e.g., 1 Cor 5:1–13), and it is carried out by a majority vote of the assembled church (2 Cor 2:6). 

Voting: Just mentioned above, Paul described a repentant individual as one once excluded by “the majority” (2 Cor 2:6). Having a clearly defined membership, the Corinthians knew their exact number in order to determine a majority vote. In a leadership situation, Luke used the verb cheirotoneō in Acts 14:23 to refer to the appointment of elders, literally meaning to “stretch out the hand” in a voting situation. The members voted their pastors into leadership, which again assumes the churches knew who their members were and who could vote. From Acts 6:3 above, the same word for appointing deacons (kathistēmi) is used of elders  in Titus 1:5, which means that elders, too, were suggested by members from among the membership, implying a clearly defined church membership. 

While an early church may not have been as formal as many churches today, there was at least an expectation of belief and living necessary for being admitted into its membership, an admission carried out by the existing membership. However formal or informal a church may be about the matter, the NT indicates that church membership must be present in some way.