The 20th century experienced a flowering of ecumenism. Four different strands—the international Christian movement, cooperation in world mission, Life and Work, and Faith and Order—developed in the early decades and, though distinctive in their emphases, later converged to form one ecumenical movement.

The modern ecumenical era began with a worldwide movement of Christian students, who formed national movements in Great Britain, the United States, Germany, Scandinavia, and Asia. In 1895 the World Student Christian Federation, the vision of American Methodist John R. Mott, was established “to lead students to accept the Christian faith” and to pioneer in Christian unity. The World Missionary Conference at Edinburgh (1910) inaugurated another aspect of ecumenism by dramatizing the necessity of unity and international cooperation in fulfilling the world mission of the church. In 1921 the International Missionary Council (IMC) emerged, bringing together missionary agencies of the West and of the new Christian councils in Asia, Africa, and Latin America for joint consultation, planning, and theological reflection. The Life and Work movement was pledged to practical Christianity and common action by focusing the Christian conscience on international relations and social, industrial, and economic problems. Nathan Söderblom, Lutheran archbishop of Uppsala, inspired world conferences on Life and Work at Stockholm (1925) and Oxford (1937). The Faith and Order movement, which originated in the United States, confronted the doctrinal divisions and sought to overcome them. Charles H. Brent, an Episcopal missionary bishop in the Philippines, was chiefly responsible for this movement, although Peter Ainslie, of the Disciples of Christ, shared the same vision and gave significant leadership. World conferences on Faith and Order at Lausanne (Switzerland; 1925), Edinburgh (1937), Lund (Sweden; 1952), and Montreal (1963) guided the process of theological consensus building among Protestants, Orthodox, and Roman Catholics, which led to approval by the Faith and Order Commission of the World Council of Churches of the historic convergence text Baptism, Eucharist, and Ministry (1982).

The World Council of Churches (WCC) is a privileged instrument of the ecumenical movement. Constituted at Amsterdam in 1948, the conciliar body includes more than 300 churches—Protestant, Anglican, Eastern Orthodox, and Oriental Orthodox—which “confess the Lord Jesus Christ as God and Saviour according to the Scriptures and therefore seek to fulfill together their common calling to the glory of the one God, Father, Son, and Holy Spirit.” Its general secretaries have been among the architects of modern ecumenism: Willem Adolph Visser ’t Hooft (Netherlands), Eugene Carson Blake (United States), Philip Potter (Dominica), Emilio Castro (Uruguay), Konrad Raiser (Germany), and Samuel Kobia (Kenya). The witness and programs of the WCC include faith and order, mission and evangelism, refugee and relief work, interfaith dialogue, justice and peace, theological education, and solidarity with women and the poor. What distinguishes the WCC constituency is the forceful involvement of Eastern and Oriental Orthodox churches and churches from the developing world. Through their active presence the WCC, and the wider ecumenical movement, has become a genuinely international community.

Roman Catholic ecumenism received definitions and momentum at the Second Vatican Council (Vatican II; 1962–65), under the ministries of Popes John XXIII and Paul VI, and through the ecumenical diplomacy of Augustin Cardinal Bea, the first president of the Secretariat for Promoting Christian Unity. The church gave the ecumenical movement new hope and language in the “Decree on Ecumenism” (1964), one of the classic ecumenical teaching documents. Another result of Vatican II was the establishment of a wide variety of international theological dialogues, commonly known as bilateral conversations. These included Roman Catholic bilaterals with Lutherans (1965), Eastern Orthodox (1967), Anglicans (1967), Methodists (1967), Reformed (1970), and Disciples of Christ (1977). Topics identified for reconciling discussions included baptism, the Eucharist, episcopacy and papacy, authority in the church, and mixed marriage. Conversations were also held with the Oriental Orthodox churches and the Assyrian Church of the East, resulting in joint statements (1992 with the Oriental Orthodox; 1994 with the Assyrian Church of the East) that resolved many of the ancient Christological disputes, although they did not result in full communion.

Critical to modern ecumenism is the birth of united churches, which have reconciled formerly divided churches in a given place. In Asia the first united churches were organized in China (1927), Thailand (1934), Japan (1941), and the Philippines (1944). The most-heralded examples of this ecumenism are the United Church of Canada (1925), the Church of South India (1947), and the Church of North India (1970). Statistics of other united churches are revealing. Between 1948 and 1965, 23 churches were formed. In the period from 1965 to 1970, unions involving two or more churches occurred in the West Indies (in Jamaica and Grand Cayman), Ecuador, Zambia, Zaire (now the Democratic Republic of the Congo), Pakistan, Madagascar, Papua New Guinea, the Solomon Islands, and Belgium. Strategic union conversations were undertaken in the United States by the nine-church Consultation on Church Union (1960) and by such uniting churches as the United Church of Christ (1957), the Presbyterian Church (U.S.A.) (1983), and the Evangelical Lutheran Church in America (1988). In Europe the Protestant Church in the Netherlands (2004) was formed from the merger of the three largest Dutch churches.

Spiritual disciplines play a key role in ecumenism, a movement steeped in prayer for unity. During the Week of Prayer for Christian Unity, celebrated every year (January 18–25), Christians from many traditions engage in prayer, Bible study, worship, and fellowship in anticipation of the unity that Christ wills.