" There is but one way in which the unity of Christians may be fostered, and that is by furthering the return to the one true Church of Christ of those who are separated from it; for far from that one true Church they have in the past fallen away. The one Church of Christ is visible to all, and will remain, according to the will of its Author, exactly the same as He instituted it. "
― Pope Pius XI , On Fostering True Religious Unity: Mortalium Animos