The movement in the Church towards the visible union of all believers in Christ. Aspirations for unity can be traced from NT times. The modern ecumenical movement may be dated from the Edinburgh Conference of 1910, though this owed much to earlier developments. It led to the establishment of the International Missionary Council and thence to the creation in 1925 of the Universal Christian Conference on Life and Work and of the first World Conference on Faith and Order which met in Lausanne in 1927. In 1948 these two bodies joined to form the World Council of Churches (q.v.).
The initiative between 1910 and 1927 came mainly from within W. Protestantism. The World Council of Churches, however, from the beginning included some E. Orthodox and Oriental Orthodox Churches. In 1961 official RC observers were for the first time permitted to attend the World Council of Churches' Third Assembly; in 1962 non-RC observers were invited to the Second Vatican Council, and in 1964 the Council's Decree on Ecumenism described members of other Churches as ‘separated brethren’ rather than as outside the Church. Various organic unions among Protestant Churches have taken place (See reunion) and the multilateral discussions conducted under the auspices of the World Council of Churches are paralleled by bilateral dialogues between world-wide organizations of different denominations. Councils of Churches at regional, national, and local levels now normally include both Orthodox and RCs. Since the late 1960s there has been a marked increase in the participation of non-Western Churches and of women. So far the Pentecostal Churches have taken little part.