Why is the union of Christ and the Church considered a mystery?

In Ephesians 5:30, Paul refers to the union between Christ and His Church as a great mystery. This mystery signifies a profound spiritual reality: believers are members of Christ's body, unified with Him in a way that transcends mere metaphor. This relationship reflects the intimate bond between Christ and His Church, where He is the head and the believers are His body. This union is vital for understanding our identity and purpose within the body of Christ and is revealed through the Holy Spirit. To truly grasp this mystery, divine revelation is necessary, as it cannot be comprehended through natural reasoning or wisdom.