The Real Definition of Church Membership by Wayne E. Ward

The foremost characteristic of this church is its dynamic unity (speaking of the church formed on the Day of Pentecost).  Membership in this community did not consist of a record book or an official letter which anyone carried.  Membership was actual presence and participation in the prayer, the witnessing, the fellowship, and the common life of the community.

That is what real membership in the church always is.  People who move away from their church community and do not promptly put their lives to work in a new church fellowship are not really members of the church in the New Testament sense of the word.  We say, "They have not moved their letter." But their "letter" is not their membership; it is simply a recommendation from brethren to brethren.  It means nothing unless it is used by actually joining the brethren.  Nor is membership simply the record of some former relationship to a congregation of Christian believers.  It is active participation in the living fellowship of the church.  

Its members shared a common life in Christ, and they were empowered by the one Holy Spirit.  This is the church that Jesus built!

(Wayne E. Ward was professor of Christian theology at Southern Baptist Theological Seminary from 1951 to 1991.  The above is a direct quote from his book "The Drama of Redemption," on page 115 written in 1966.)