The first thing we must see clearly is that we are placed in the body of Christ by the Spirit. "For by one Spirit we were all baptized into one body--whether Jews or Greeks, whether slaves or free--and have all been made to drink into one Spirit" (I Corinthians 12:13). This strikes at the heart of the idea that we can "join" the body of Christ. Only God can join us to what He is doing, and He does this by bringing us into His life. This is a spiritual transaction with God, whereby He comes to abide within each member as He promised. "If anyone loves me, he will keep my word; and my Father will love him, and we will come to him and make our home with him" (John 14:23). This transaction is a birth into God, and brings us into connection with the Head, which is Christ. We can now call Christ Lord, because we are in a place in which we can be controlled by the Spirit. "...No one can say that Jesus is Lord except by the Holy Spirit" (I Corinthians 12:3).
The body of Christ is what it is because Jesus is Lord of every member, and every member is responding to the direction of the Head. Without that connection, there is no body. This must be spiritual reality, not just a mental assent. "But why do you call me Lord, Lord, and do not do the things which I say?" (Luke 6:46) This gives us a beginning definition of the body of Christ: those that do the will of God. If we are not those who can say we are doing the will of God, obeying the Head, we cannot say that we are part of His body. We may be a part of some religious organization, but the body of Christ is the body of which Christ is the Head, and that means control in reality.
Some may try to separate this birth into the body of Christ from salvation, but a careful and honest search of the Word of God will not allow these to be separated. Salvation is not something we receive apart from God but in union with God, by coming under His authority. We are saved by His life (Romans 5:10). He is our salvation in every way. He saves us, not by telling us how to live, but by becoming our life. We are brought into His life by repentance and faith. "For as many as are led by the Spirit of God, these are the sons of God" (Romans 8:14). Every other aspect of the body of Christ is based on the fact that each member is in connection with the Head by the Spirit and is being led by the Spirit. Let each member know for certain that he is alive to God and attached to the Head by the Spirit. This is our beginning place in God.