The highest moral laws in the secular west today are toleration and non-judgmentalism. The very ideas of inclusion and exclusion in any context make people think of elitism, xenophobia, or racism.
Still, church membership is an idea with deep biblical roots:
•The garden of Eden had an inside and an outside (Gen. 2:8, 3:24).
•Noah’s ark had an inside and an outside (Gen. 7:16).
•The people of God in the wilderness had to be ceremonially clean to remain inside the camp and not to be cast outside (Lev. 13:46, Num. 5:3).
•The nation of Israel was to have an inside and an outside, as represented by food laws, festivals, political boundaries, places of worship, and even strong provisions against inter-marriage (Lev. 11, Lev. 23, Josh. 14-21, 1 Kings 8, Deut. 7:3).
•In the New Testament, the ethnic and cultural boundary markers of the Old Testament are exploded. But a clear, bright line remains between those who are God’s people and those who are not, as indicated by their profession of faith and the visible fruit of their lives, as well as by the practices of baptism and the Lord’s Supper (Rom. 6:3-4, 1 Cor. 11:17-34, Gal. 3:27). Baptism and the Lord’s Supper are what give substance and shape to a church’s membership. There is an inside (those who have been baptized and are welcomed to the Lord’s table) and an outside (those who are not).
•In 1 Corinthians 5:12-13 Paul challenges the Corinthian church to judge those who are inside the church and leave judging outsiders to God. How did the Corinthians know who was in and who was out? They knew whom Paul was talking about because some people had formally, publicly identified themselves with the church in Corinth while the rest of the city had not. Those who had were inside the church. They were the church’s members. Those who hadn’t were not.
Taken from 9marks.org