By the 20th century the ecumenical movement had become perhaps the single most prominent feature of contemporary Christian history. Doctrinal conversations were held at the multilateral level under the heading of Faith and Order and at the bilateral level between particular pairs among the global confessional families or communions. They often started as what might be called “comparative symbolics”—the matching of existing confessional statements—but they then moved into a concentration on the dogmatic topic in hand.

Although the ecumenical patriarchate of Constantinople, which holds honorary primacy within Eastern Orthodoxy, took a significant first step by its proposal of a “league of churches” in 1920, the ecumenical movement was largely Protestant in its origins. The Roman see suspected “religious indifferentism” (as indicated in Pius XI’s encyclical of 1928, Mortalium animos [“On Religious Unity”]) and was hesitant to join the movement. But pioneering efforts in “spiritual ecumenism,” followed by mid-century convergences at the scholarly level, prepared for the official entrance of the Roman Catholic Church on the ecumenical scene with the holding of the Second Vatican Council (Vatican II; 1962–65). When, after some 50 years as the principal carrier of the ecumenical banner, the World Council of Churches suffered some decline, Pope John Paul II in his encyclical Ut unum sint (1995; “That They May Be One”) reaffirmed the “irrevocable commitment” of the Catholic church to the ecumenical cause of Christian unity for the sake of obedience to Christ’s will and the truth and spread of the gospel: “that they may be one, that the world may believe” (John 17:17–23), and “all for the glory of the Father,” as John Paul noted, summarizing the meaning of the gospel passage.

The ecumenical spirit even spread to relations between Catholic and Orthodox churches on one side and the Oriental Orthodox churches on the other. Some of the Oriental Orthodox churches, most notably the Armenian Apostolic Church, had been founding members of the World Council of Churches in 1948. Beginning in the mid-20th century, the Eastern Orthodox churches and the Roman Catholic Church each undertook independent efforts at dialogue to mend the ancient rift that followed the Council of Chalcedon. The Eastern Orthodox and the Oriental Orthodox churches reached theological agreement that they shared the same Christological faith despite terminological differences over “one nature” or “two natures.” Somewhat similarly, the Roman see reached agreement with the Oriental Orthodox churches that the ancient conflict over perceived differences in Christology had been largely a “verbal” matter, and the two parties even approved mutual sacramental ministry in cases of pastoral emergency. Since 1970 the Vatican and the leaders of four of the six Oriental Orthodox churches (Armenian Apostolic, Syriac Orthodox, Coptic Orthodox, and Malankara Syrian [Indian]) have issued joint declarations about a shared Christian vision, and the Catholic church has held joint meetings with all six churches every January since 2004. Individual Oriental Orthodox churches have engaged in occasional dialogues with the Assyrian Church of the East and with various Protestant bodies. Despite such overtures, however, there remain traditionalists on each side who reject ecumenism.

The Filioque dispute between East and West, which originated in the 8th century, seemed on the way to resolution through theological work associated with the Faith and Order Commission study, Spirit of God, Spirit of Christ (1981): mutually acceptable understandings were approached through formulations such as “the Spirit proceeds from the Father through the Son” or “from the Father of the Son.” The Catholic and Orthodox churches began a bilateral dialogue after Vatican II over a broader range of dogmatic topics, although it was recognized that each of the two churches would have to modify its claim to constitute the one Church of Christ if they were to be reunited.

The Faith and Order Commission study that attracted the greatest degree of participation from the widest range of churches was Baptism, Eucharist, and Ministry (1982), whose initial reception was remarkably positive. On the touchstone question of the Eucharist, the study affirmed the “real, living, and active presence” of Christ but hardly settled the centuries-old controversies over the manner of that presence in relation to the bread and wine.

On All Saints’ Eve, 1999, the Joint Declaration on the Doctrine of Justification was signed in Augsburg, Germany, by representatives of the Pontifical Council for Promoting Christian Unity and the Lutheran World Federation. By declaring that the mutual condemnations of the 16th century did not apply to the teaching on justification as now stated together (with tolerable nuances of detail on either side), Lutherans and Catholics proclaimed what may have been the key issue of the Reformation settled, even if other doctrinal and institutional matters needed to be resolved before full reconciliation could take place.

In multilateral terms, Pope John Paul II, in the encyclical Ut unum sint, noted the considerable ecumenical progress made in the second half of the 20th century on doctrinal questions and then listed five topics that required further study: (1) “the relationship between Sacred Scripture, as the highest authority in matters of faith, and Sacred Tradition, as indispensable to the interpretation of the Word of God,” (2) “the Eucharist, as the Sacrament of the Body and Blood of Christ, an offering of praise to the Father, the sacrificial memorial and Real Presence of Christ, and the sanctifying outpouring of the Holy Spirit,” (3) “ordination, as a sacrament, to the threefold ministry of the episcopate, presbyterate and diaconate,” (4) “the Magisterium of the Church, entrusted to the Pope and the Bishops in communion with him, understood as a responsibility and an authority exercised in the name of Christ for teaching and safeguarding the faith,” and (5) “the Virgin Mary, as Mother of God and Icon of the Church, the spiritual Mother who intercedes for Christ’s disciples and for all humanity.” John Paul II invited “the leaders of other churches and their theologians” to engage with him “in a patient and fraternal dialogue” on the claims of the primatial Roman see to a universal ministry of unity. He felt his own responsibility “in acknowledging the ecumenical aspirations of the majority of the Christian Communities and in heeding the request made of me to find a way of exercising the primacy which, while in no way renouncing what is essential to its mission, is nonetheless open to a new situation.”

The challenge facing the established churches in remaining committed to their traditional mission while responding to changing circumstances may become further evident in the 21st century with the global burgeoning of Pentecostal and charismatic churches that have had little to do with existing institutions but display many features of historical Christianity. (See below Ecumenism.