Unitatis redintegratio (Latin for "Restoration of unity") 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. Its title is taken from the opening words of the Latin text. The opening of the document's English translation is: "The restoration of unity among all Christians is one of the principal concerns of the Second Vatican Council."
Unitatis Redintegratio calls for the reunion of Christendom and is similar to previous calls for unity by Pope Leo XIII in the 1894 encyclical Praeclara gratulationis publicae. However, Unitatis articulates a different kind of ecclesiology than Praeclara. It focuses on the unity of the people of God and on separate Christian brethren rather than insisting according to the classical formulation that schismatics must return to the fold under the unity of the Vicar of Christ.
Unitatis acknowledges that there are serious problems facing prospects of reunion with Reformation communities that make no attempt to claim apostolic succession as the Anglican communion does. Ecclesial communities that adhere to Calvinism are a particularly challenging case because they and Catholicism 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 as articulated by the council of Nicea in the early Church.
Traditionalist Catholics argue that this document contradicts the teachings of popes who preceded the Second Vatican Council and gives a false representation of the unity of the Catholic Church. They cite documents such as Mortalium Animos by Pope Pius XI, which addresses statements similarly expressed in Unitatis Redintegratio. Pope Pius XI considered the position that the Church of Christ can be divided into sections and that the Unity of the Church has not been achieved as a false opinion. Considering these notions, Pius XI continued
"[T]he Apostolic See cannot on any terms take part in [non-Catholic] assemblies, nor is it anyway lawful for Catholics either to support or to work for such enterprises; for if they do so they will be giving countenance to a false Christianity, quite alien to the one Church of Christ. Shall We suffer, what would indeed be iniquitous, the truth, and a truth divinely revealed, to be made a subject for compromise? For here there is question of defending revealed truth." 
The numbers given correspond to the section numbers within the text.
- Introduction (1)
- I. Catholic Principles on Ecumenism (2-4)
- II. The Practice of Ecumenism (5-12)
When considering how the Church may allow for "common worship", which must never be used "indiscriminately", it is for "local episcopal authority, unless otherwise provided for by the Bishops' Conference according to its statutes, or by the Holy See" to determine the course to follow, making due provision for specific "circumstances of time, place, and persons". (8)[a]
- III. Churches and Ecclesial Communities Separated from the Roman Apostolic See (13-24)
- III 1. The Special Consideration of the Eastern Churches (14-18)
- III 2. Separated Churches and Ecclesial Communities in the West (19-24)
- In 2018, Cardinal Kasper cited section 8 as authorization for the German Bishops' Conference to allow certain non-Catholics, when married to Catholics and sharing certain beliefs of the Catholic faith, to participate in Communion.
- Kasper, Walter (11 November 2004). "The Decree on Ecumenism, Read Anew After Forty Years". Pontifical Council for Promoting Christian Unity. Retrieved 13 May 2018.
- Mortalium Animos, Pius XI. http://www.papalencyclicals.net/pius11/p11morta.htm
- Tornielli, Andrea (13 May 2018). "'Il Concilio e due encicliche ammettono casi di eucaristia ai protestanti'". La Stampa (in Italian). Retrieved 13 May 2018.