Biblical basis of church membership
In the letters written to local churches in the New Testament, those who belong to the churches are referred to as “members of Christ” (1 Corinthians 6:15), “members of his household” (Ephesians 2:19), “members of one body” (Ephesians 3:6, 4:25 & Colossians 3:15) and “members of his body” (Ephesians 5:30).
In the book of Acts, phrases such as, “the whole congregation” (6:5), “the church in Jerusalem” (8:1), “the disciples in Jerusalem” (9:26), “in every church” (14:23), “the whole church” (15:17), and “the Elders of the church in Ephesus” (20:17), all suggest a recognisable local church membership with well-defined boundaries.
There is also evidence in the New Testament that, just as there was a list of widows eligible for financial support (1 Timothy 5:9), there must have also been a list of members that grew as people were saved (Acts 2:41, 47; 5:14; 16:5).
The need for a defined list of members in each local church is implied by the command to church elders to care specifically for those for whom the Holy Spirit has made them accountable (Acts 20:28), and by the command for members to submit to their leaders (Hebrews 13:17). It works both ways: elders are accountable to the Lord for the spiritual care of a defined list of members in the local church; church members are to submit to the elders of the specific local church to which they belong as members.
When believers moved to another city, their previous church wrote a letter of commendation to their new church (Acts 18:27; Romans 16:1; 1 Corinthians 16:3; Colossians 4:10; cf. 2 Corinthians 3:1-2).
The ultimate sanction of church discipline, in the case of serious, unrepentant sin in the church, is the removal of someone from membership (1 Corinthians 5:12-13 & Matthew 18:15-17). This requires that churches must be clear about who is inside, and who is outside, the local church.
Being joined to Christ, every member is also part of his body, his church. The New Testament does not envisage a Christian who is a loner and not part of any local church. Fellowship, learning, service and relationships in a committed, local church context are the natural outflow of the new birth and a necessary sign of spiritual life. Church membership identifies those who are committed to the local church and subject to its care and discipline.
(While not every local church today has a formal membership list, or practices membership in exactly the same way, all biblically faithful churches must have some means of identifying who is committed to their church and subject to its care and discipline.)