Membership is a decision to unselfishly give to the spiritual needs of others. Commitment always builds character. It defines who can be counted on. Every team has a roster. Every school must have an enrollment.
Every business has a payroll.
Every army has an enlistment. Even our country takes a census and requires voter registration. Membership identifies who really has made a commitment to be in our faith-family. The New Testament places a major emphasis on the need for Christians to be accountable to one another for spiritual growth. You cannot be as accountable when you are not committed to any specific church family.