19 In the great upheaval which began in the West toward the end of the Middle Ages, and in later times too, Churches and ecclesial Communities came to be separated from the Apostolic See of Rome. Yet they have retained a particularly close affinity with the Catholic Church as a result of the long centuries in which all Christendom lived together in ecclesiastical communion. However, since these Churches and ecclesial Communities, on account of their different origins, and different teachings in matters of doctrine on the spiritual life, vary considerably not only with us, but also among themselves, the task of describing them at all adequately is extremely difficult; and we have no intention of making such an attempt here. Although the ecumenical movement and the desire for peace with the Catholic Church have not yet taken hold everywhere, it is our hope that ecumenical feeling and mutual esteem may gradually increase among all men. It must however be admitted that in these Churches and ecclesial Communities there exist important differences from the Catholic Church, not only of a historical, sociological, psychological and cultural character, but especially in the interpretation of revealed truth. To make easier the ecumenical dialogue in spite of these differences, we wish to set down some considerations which can, and indeed should, serve as a basis and encouragement for such dialogue.
Source: Unitatis Redintegratio (Vatican.va)