Catechism of the Catholic Church §855 — excerpt
The Church's mission stimulates efforts towards Christian unity. Indeed, "divisions among Christians prevent the Church from realizing in practice the fullness of catholicity proper to her in those of her sons who, though joined to her by Baptism, are yet separated from full communion with her.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: RMiss 50 · UR 4 # 8