Aeterna Dei Sapientia §40

⁴⁰ But mark this well: unless the faithful remain bound together by the same ties of virtue, worship and sacrament, and all hold fast to the same belief, they cannot be perfectly united with the Divine Redeemer, the universal Head, so as to form with Him one visible and living body. "A whole faith," says St. Leo, "a true faith, is a mighty bulwark. No one can add anything to it, no one can take anything away from it; for unless it is one, it is no faith at all." (35)