"For just as the body is one and has many members, and all the members of the body, though many, are one body, so it is with Christ. For in one Spirit we were all baptized into one body—Jews or Greeks, slaves or free—and all were made to drink of one Spirit." - 1 Corinthians 12:12-13
Church membership is important because true churches are a visible representation on earth of Christ's invisible kingdom. As we unite ourselves to him in saving faith we are also united to his body - we are part of the church invisible. For our benefit, God has given the visible church to which we unite ourselves through vows. He does this for our good and commands us not to take the attitude that we do not need the rest of the body (1 Corinthians 12:21-26).
This is more than just attending church (though that is important: Hebrews 10:24-25)! In Christ, we are all vital parts of a larger body that exists to serve and be served as a testimony of Christ's goodness and salvation in the world.
It's important to join a church where the doctrines and practices are such that you are able to promote both the peace and purity of that body. To that end, we require members to affirm the Nicene Creed without exception, to have read the Heidelberg Catechism to understand what our pastor and elders believe, and to affirm the membership vows in truth and clear conscience. Learn more about why we use creeds and catechisms here.
If you are interested in learning more about membership at Covenant of Grace please speak to any of the elders!