Mission - a requirement of the Church's catholicity
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. Furthermore, the Church herself finds it more difficult to express in actual life her full catholicity in all its aspects."
Source: Catechism of the Catholic Church (Vatican.va)