Careful practices of membership and discipline are meant to mark off the church from the world and thereby define and display the gospel.
Churches which practice no formal membership and discipline at least make it more difficult for the believers who are part of it to follow Christ and more difficult for those elders to know for whom they are to given an account (Heb. 13:17). In fact, I would go a step further and say that churches which practice no self-conscious membership are in sin since Christians cannot follow basic biblical commands without it.
The Church, xxiii