Union of Christendom, 1911 Catholic Encyclopedia: "The
Catholic Church is by far the largest, the most widespread, and the most ancient of
Christian communions in the world, and is moreover the mighty trunk from which the other communions claiming to be
Christian have broken off at one time or another. If, then, we limit the application of the term
Christendom to this, its most authentic expression, the unity of
Christendom is not a lost ideal to be recovered, but a stupendous reality which has always been in stable possession. For not only has this
Catholic Church ever taught that unity is an
essential note of the
true Church of
Christ, but throughout her long history she has been, to the amazement of the world, distinguished by the most conspicuous unity of
faith and government, and this notwithstanding that she has at all times embraced within her fold nationalities of the most different temperaments, and has had to contend with incessant oscillations of
mental speculation and political power. Still, in another and broader sense of the term, which is also the more usual and is followed in the present article,
Christendom includes not merely the
Catholic Church, but, together with it, the many other religious communions which have either directly or indirectly, separated from it, and yet, although in conflict both with it and among themselves as to various points of
doctrine and practice agree with it in this: that they look up to our Lord
Jesus Christ as the Founder of their Faith, and claim to make His teaching the rule of their lives. As these separated communities when massed together, indeed in some cases even of themselves, count a vast number of
souls, among whom many are conspicuous for their religious earnestness, this extension of the term
Christendom to include them all has its solid justification. On the other hand, if it is accepted, it becomes no longer possible to speak of the unity of
Christendom but rather of a
Christendom torn by divisions and offering the saddest spectacle to the eyes. And then the question arises: Is this
scandal always to continue? The
Holy See has never tired of appealing in season and out of season for its removal but without meeting with much response from a world which had learnt to live contentedly within its sectarian enclosures. Happily a new spirit has lately come over these dissentient
Christians, numbers of whom are becoming keenly sensitive to the paralyzing effects of division and an active reunion movement has arisen which, If far from being as widespread and solid as one could wish, is at least cherished on all sides by devout
minds ... We may safely leave to the
Providence of God to determine what course the present reunion movement shall ultimately take, and meanwhile we may emphasize the substantial point that
Catholics and other reunionists have in common: their mutual desire to see the barriers that separate them removed. They can co-operate, too, in working for the good cause in useful ways without any surrender of their own principles. For they can cultivate friendly personal relations, to the formation of which it will greatly contribute if they can work together for objects, social or otherwise, as to the value of which they are agreed. There is a special value in the personal friendships thus formed, for they tend to dissolve the obstacles which come from sheer misunderstandings and the animosities that these engender. And they can further co-operate for the removal of these same obstacles by positive efforts to understand one another correctly, particularly by the others seeking and the
Catholics, if they are competent, showing a readiness to give simple explanations of the
true character of their
beliefs and practices."
http://www.newadvent.org/cathen/15132a.htmHoly Office Decree on the Ecuмenical Movement: "Now in many parts of the world, as a result of various external events and changes of views on the part of people, but especially in consequence of the common prayers of-the faithful through the grace of the Holy Spirit, there has grown constantly in the minds of many persons separated from the Catholic Church the desire for a return to unity on the part of all who believe in the Lord Christ. To the children of the Church this is surely a cause of true and holy joy in the Lord, and at the same time an invitation to help all those who sincerely seek the truth, by earnest prayer to God imploring for them the grace of light and strength.
However, some of the initiatives that have hitherto been taken by various individuals or groups, with the aim of reconciling dissident Christians to the Catholic Church, although inspired by the best of intentions, are not always based on right principles, or if they are, yet they are not free from special dangers, as experience too has already shown. Hence this Supreme Sacred Congregation, which has the responsibility of conserving in its entirety and protecting the deposit of the faith, has seen fit to recall to mind and to prescribe the following ... It should be made clear to them that, in returning to the Church, they will lose nothing of that good which by the grace of God has hitherto been implanted in them, but that it will rather be supplemented and completed by their return."
On Dec. 20th 1949.
https://www.ewtn.com/catholicism/library/on-the-ecuмenical-movement-2070