Why is church membership so important?

9 Marks ministries gives these 3 great reasons to have a well-defined and clarified church membership. Check out their website at 9marks.org

1. Showing the world who is a Christian and who is not. The clearer the line between Christian and non-Christian, the clearer the gospel will be.
2. Demonstrating that the gospel is a life-transforming message. When a church is full of people who have been set free from slavery to sin and now are slaves of righteousness (Rom. 6:15-23), it will show the world that the gospel really is the power of God for the salvation of everyone who believes (Rom. 1:16), and that this salvation necessarily involves living a radically new life of holiness and obedience to God.
3. Displaying the power of the gospel to unite people. When a church is composed of Christians who are united in faith and love, that church demonstrates that the gospel has power to unite people who would otherwise be divided. It also shows that the gospel is not limited to any ethnic or social group, but is rather a universally relevant message that calls all people to repent and offers eternal life to everyone who believes in Christ.