Acts 5.13 tells the story of how after the deaths fo Ananias and Sapphira people recognized that beign associated with the church was no trifling matter. God was taking this thing seriously. And so many were afraid to "join" them.
What is that joining all about? The context tells us that these folks were still associated with the church but they hung out on the fringe. What God was asking for from his church was a full on radical commitment. Association and commitment! These days the most radical thing you see in church is a soul patch on some dudes face. Christ deserves and demands more.
People don't see the importance of church membership these days because we have reduced it to signing a card. Or attending a set of classes. Church membership is a fearless commitment to the cause of Christ and the vision of a local assembly. I think that commitment is radical by todays standards! Many won't commit to a single thing in their life and they are the poorer for it. Rick Warren says that people who attend church and don't commit to membership are "Spiritual Adulterers" - equating the no-commitment to living together without committing to marriage.
Church membership is a two way commitment too. The Body of Members commit to watch out for one another (speak the truth, encourage, get involved, etc.) and to help one another (food pantry, minitries of helps, help with a bill now and again) as well as provide teaching, opportunities for growth and ministry, and service. God seems to move in amazing ways when people are in committed relationship.
So before you sign the card... ask yourself if you are ready to get radically committed.