Every follower of Jesus is a member of the body of Christ. Local church membership is the public acknowlegment and application of this spiritual reality. As followers of Jesus, we are “members of the household of God” (Ephesians 2:19), and it is our responsibility to be the Church as a member of His body. As each member plays their part, God uses us to build His body by His Spirit.
For as in one body we have many members, and the members do not all have the same function, so we, though many, are one body in Christ, and individually members one of another. Romans 12:4-5