What does it mean to be in Him? It means to be in Christ. In the Bible Jesus is the Name given to God becoming flesh (Matthew 1:21-23; John 1:14). Christ refers to the divinity of Jesus-that which is God Himself. 2 Corinthians 5:17 states: "Therefore if any man be in Christ, he is a new creature (creation):old things are passed away; behold all things are become new." (King James Version). The literal Greek states that this new creature (creation) is a new species of being that has never before existed. The moment you were born again God created a new "you" in your heart (not your head). There's a new person living in you-a new creation that has the same Life of God in it that Jesus had on the earth. As a Christian are a part of God's family, His Church, which Jesus is the Head of (Ephesians 1:22;5:23).
Jesus is the Head of the Church. Not man or any organization, only Jesus. Since Jesus is the Head of the Church, the same Church that we're now in, Who-or what-are we? We are the Body of Christ (Ephesians 1:22,23; 5:30-32).