The Church is the people of God, the body of Christ. All who trust in Christ for salvation automatically become part of the Church, whether they like it or not.
However, not all who go to church services, who become members of a church, are part of the Church. This is an important distinction. John Calvin referred to the public, institutional church as the visible church. We can see who attends services. We can proclaim our membership of a particular flock or denomination.
The invisible church is known only to God. It is “His prerogative”, using Calvin’s words, to know who is the elect, from beginning to end of creation. And in light of this, we are to withhold judgment. We can, however, recognize members of this invisible church by their confession of faith, partaking of the sacraments, and their profession of Christ. We know a tree by its fruit.
Recognizing this distinction between the visible and invisible Church breeds humility. Authority over the Church is God’s. No man can be the gatekeeper to the body of Christ, letting those he deems worthy in or out. Some people are content to do the religion thing, but not surrender to Christ. God will judge their hearts. It is our job to serve within the Church. We are aware that those who seem far from God may be called to him at some point in their lives. God will call those he’s elected.
We are also aware that God has placed upon man the important task of shepherding his Church. We are to disciple and to encourage the visible church. We are to guard the visible church, and by guard, I mean we are not to open membership to anyone and everyone, but to set forth the requirements of scripture for salvation: namely, confession of faith in Jesus Christ.
The early church was extremely particular in its membership requirements. During the startup of the visible church, in the first several hundred years after Jesus lived on earth, heresies were rampant, and the church fathers were correct in being protective of the Gospel. Requirements for church membership included a discipleship period of several years and a renunciation of former ways of living. The sacrament of the Lord’s Supper was closed to church members only, whereas the teaching and singing components of worship were open to the public.