Ecumenism (//), also spelled oecumenism, is the concept and principle in which Christians who belong to different Christian denominations work together to develop closer relationships among their churches and promote Christian unity. The adjective ecumenical is thus applied to any interdenominational initiative that encourages greater cooperation between Christians and their churches.
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The fact that all Christians belonging to mainstream Christian denominations profess faith in Jesus and receive baptism according to the Trinitarian formula is seen as being a basis for ecumenism and its goal of Christian unity. Ecumenists cite John 17:20–23 as the Biblical grounds of striving for church unity, in which Jesus prays that Christians "may all be one" in order "that the world may know" and believe the Gospel message.
In 1920, the Ecumenical Patriarch of the Eastern Orthodox Church, Germanus V of Constantinople, wrote a letter "addressed 'To all the Churches of Christ, wherever they may be', urging closer co-operation among separated Christians, and suggesting a 'League of Churches', parallel to the newly founded League of Nations". In 1937, Christian leaders from mainstream Christian Churches thus resolved to establish the World Council of Churches to work for the cause of Christian unity; it today includes most major traditions of Christianity as full members, including the Assyrian Church of the East, the Old Catholic Church, the Oriental Orthodox Churches, the Lutheran Churches, the Anglican Communion, the Baptist Churches, the Mennonite Churches, the Methodist Churches, the Moravian Church, the Pentecostal Churches and the Reformed Churches, as well as almost all jurisdictions of the Eastern Orthodox Church; the Roman Catholic Church participates as an observer, sending delegates to official gatherings.
Many regional councils affiliated with the World Council of Churches, such as the Middle East Council of Churches, National Council of Churches in Australia and Christian Churches Together, work for the cause of Christian unity on the domestic level, with member denominations including the Oriental Orthodox Churches, Lutheran Churches, Catholic Church, Eastern Orthodox Churches, Methodist Churches, Anglican Communion, Reformed Churches, among others.
The terms ecumenism and ecumenical come from the Greek οἰκουμένη (oikoumene), which means "the whole inhabited world", and was historically used with specific reference to the Roman Empire. The ecumenical vision comprises both the search for the visible unity of the Church (Ephesians 4:3) and the "whole inhabited earth" (Matthew 24:14) as the concern of all Christians. In Christianity, the qualification ecumenical was originally and still is used in terms such as "Ecumenical council" and "Ecumenical Patriarch", in the meaning of pertaining to the totality of the larger Church (such as the Catholic Church or the Eastern Orthodox Church) rather than being restricted to one of its constituent local churches or dioceses. Used in this sense, the term carries no connotation of re-uniting the historically separated Christian denominations but presumes a unity of local congregations in a worldwide communion.