Because we live in a casual and breezy age, many Christians are unfamiliar with the idea of a covenanted church membership. CREC churches usually have a formal membership roster, and for some this may require a brief explanation.
The biblical basis for this is found in Hebrews 13:7,17. Verse 7 speaks of Christian “rulers,” who have taught the Word of God, and who have lived lives worthy of imitation. Now obviously, in order to obey someone, you have to know who they are. To hear them you have to be within earshot, and to imitate them, you have to know them and their families. So for members of the congregation, it is necessary to know the roster of their elders — otherwise obedience to them is an incoherent duty, impossible to fulfill. In addition to this idea of submission and obedience, verse 17 shows us the specific responsibilities that extend in the other direction. Those who have the rule watch out for individual souls, and they do so as ones who must give account. One of the things that those who will give an account must do is actually count. If a father goes out to the park with the kids, when he returns, and mom asks him if he has all of them with him, she will not be satisfied with “more or less” for an answer. Verse 17 requires some kind of membership roster. “Be thou diligent to know the state of thy flocks, and look well to thy herds” (Prov. 27:23)
While attachment to a particular congregation is important, it is not important in the same fixed way that a marriage is, for example. A man might lawfully leave a congregation to take a job in another part of the country, but it would not be lawful for him to desert his family for that same job. Because the CREC is not sectarian, we also think it is legitimate for someone to transfer from one church to another in the same community — if the attitude is not schismatic, then the action certainly is not. Church membership is simply a way for members and elders both to take some form of orderly responsibility. For us, it is not a matter of ownership (the “lording over” prohibited to elders in 1 Pet. 5:3) or isolation from other believers (the partisan spirit prohibited to followers in 1 Cor. 3:4).
The CREC stands for the Communion of Reformed Evangelical Churches. You can find out more here or here.