I have an article on church membership up at Ref21. Here’s a portion,
Yet, Paul grounds the two metaphors in the same reality: Because of the gospel, Jews and Gentiles alike are sus-soma, of the same body (Ephesians 3:6). The gospel that God reveals shows that scattered individuals are united together by the Spirit under Christ, either with the metaphor of Jesus as the head of his body (Ephesians 1:22-23) or the household cornerstone (Ephesians 2:20). By the Holy Spirit, Christians are made members of the household of God, because we share in the same body, that is, we are all united to Christ. This mele, union in Christ, is central to the mystery of the gospel relationship of Christ and the church (Ephesians 5:29-32). Salvation is Jesus joining with his people and his people being joined to him. By being his members, the Christian is a participant in the divine nature (2 Peter 1:4) because Jesus abides in us and we in him (John 14). That is what church membership is, ontologically.