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Unitatis Redintegratio is the Second Vatican Council's Decree on Ecumenism. It was passed by a vote of 2,137 to 11 of the bishops assembled and was promulgated by Pope Paul VI on 21 November 1964. The title in Latin means "Restoration of Unity" and is from the first line of the decree, as is customary with major Catholic documents (see incipit).
The numbers given correspond to the section numbers within the text.
- Introduction (1)
- Catholic Principles on Ecumenism (2-4)
- The Practice of Ecumenism (5-12)
- Churches and Ecclesial Communities Separated from the Roman Apostolic See (13-24)
- The Special Consideration of the Eastern Churches (14-18)
- Separated Churches and Ecclesial Communities in the West (19-24)
Policy on the Eastern Orthodox and Oriental Orthodox 
Unitatis Redintegratio calls for the reunion of Christendom and so it is not terribly different from previous calls for unity by Pope Leo XIII in the 1894 encyclical Praeclara Gratulationis Publicae. However, the document articulates a different kind of ecclesiology than Praeclara, focusing on the unity of the people of God and on separate Christian brethren instead of a classical call for schismatics to return to the fold under the unity of the Vicar of Christ.
Reformation communities 
The document acknowledges that there are serious problems facing prospects of reunion with Reformation communities that make no attempt to claim apostolic succession such as the Anglican communion does. Ecclesial communities that adhere to calvinism are a particular case because they often have important doctrinal differences on key issues such as ecclesiology, liturgy and mariology. Other communities have insoluble doctrinal differences with Catholic Christianity because their theology of the Holy Trinity is manifestly incompatible with the doctrine of the council of Nicea in the early Church. That these serious problems are a barrier to salvation is clarified in the 2004 Vatican document, "The Decree on Ecumenism, Read Anew after Forty Years".
Separated brethren 
First officially used by the Roman Catholic Church in the Unitatis Redintegratio, "Separated brethren" is a term sometimes used by the Roman Catholic Church and its clergy and members to refer to baptized members of other Christian traditions. Though also applied to Christians of the Eastern Orthodox and Oriental Orthodox churches, the term is more often used about Protestants and Anglicans. The phrase is a translation of the Latin phrase fratres seiuncti.
Before the Second Vatican Council, per the pronouncements of the Council of Trent, the Roman Catholic Church officially referred to Protestants and other non-Roman Catholic Christians as "heretics" not having hope of salvation outside of the "Church of Rome". After the Second Vatican Council, however, "that habit of unthinkingly hurling accusations of heresy at Protestants pretty much died out". Since at least the mid-1990s, the term has often been replaced by Roman Catholic officials with phrases such as "other Christians".
At least one Roman Catholic writer does not consider Mormons and members of some other religious groups to be separated brethren. Among the groups not considered to be separated brethren are "Jews, Mormons, Christian Scientists, Muslims, Buddhists, and other groups." By the 21st century, within the Roman Catholic faith, Jews are described as and considered elder brothers in the faith.
- Kroll, Paul (October/November 2007). "Church History Corner: Vatican II and the Future of Church Unity". Christian Odyssey (Glendora, CA: Worldwide Church of God) 3 (5): 18–19. Retrieved 2010-06-21.
- Hardon, John (1980). "Separated Brethren". Modern Catholic Dictionary (CatholicCulture.org ed.). Doubleday. ISBN 978-0-385-12162-0. Retrieved 2010-06-18.
- Wells, Christopher (2009). "The Singular Grace of Division's Wound". Ecclesiology (Leiden, Netherlands: Brill) 5 (1): 10. doi:10.1163/174553108X378468. Retrieved 2010-06-18.
- Oakes, Edward T. (December 19, 2007). "On the Square: Are Protestants Heretics?". First Things Online. New York: Institute on Religion and Public Life. Retrieved 2010-07-09.
- Wells, Christopher (2009). "The Singular Grace of Division's Wound". Ecclesiology (Leiden, Netherlands: Brill) 5 (1): 10–11. doi:10.1163/174553108X378468. Retrieved 2010-06-18.
- Whalen, William Joseph (2002). Revised: Separated Brethren: A Review of Protestant, Anglican, Eastern Orthodox & Other Religions in the United States. Huntington, Ind.: Our Sunday Visitor. p. 9. ISBN 978-1-931709-05-7. Retrieved 2010-06-16.
- Unitatis Redintegratio - Text of document
- The Decree on Ecumenism, Read Anew After Forty Years - text of 2004 clarification document