The Church - mystery of men's union with God
It is in the Church that Christ fulfills and reveals his own mystery as the purpose of God's plan: "to unite all things in him." St. Paul calls the nuptial union of Christ and the Church "a great mystery." Because she is united to Christ as to her bridegroom, she becomes a mystery in her turn. Contemplating this mystery in her, Paul exclaims: "Christ in you, the hope of glory."
Source: Catechism of the Catholic Church (Vatican.va)