91 Venerable Brethren, in Our exposition of this mystery which embraces the hidden union of us all with Christ, We have thus far, as Teacher of the Universal Church, illumined the mind with the light of truth, and Our pastoral office now requires that We provide an incentive for the heart to love this Mystical Body with that ardor of charity which is not confined to thoughts and words, but which issues in deeds. If those who lived under the Old Law could sing of their earthly city: "If I forget thee, O Jerusalem, let my right hand be forgotten; let my tongue cleave to my jaws if I do not remember thee, if I make not Jerusalem the beginning of my joy," how much greater then should be the joy and exultation that should fill our hearts who dwell in a City built on the holy mountain of living and chosen stones, "Jesus Christ himself being the chief cornerstone." For nothing more glorious, nothing nobler, nothing surely more honorable can be imagined than to belong to the One, Holy Catholic, Apostolic and Roman Church, in which we become members of One Body as venerable as it is unique; are guided by one supreme Head; are filled with one divine Spirit; are nourished during our earthly exile by one doctrine and one heavenly Bread, until at last we enter into the one, unending blessedness of heaven.
Source: Mystici Corporis Christi (Vatican.va)