That they all may be one in the context of "Ecumenical"

The Catholic Church and Eastern Orthodox Church broke communion during the East–West Schism of 1054. While an informal divide between the East and West existed prior to the split, these were internal disputes, under the umbrella of the recognised “one, holy, catholic and apostolic Church” of the Nicene Creed. It is only after the formal schism of the 11th century that two, distinct churches are seen to exist, and thus commence relations. The split has, on both sides, been immensely lamented, for it defeats the exhortation of Jesus Christ “that they may all be one" (John 17:21). The anguish over the past has spurred both sides, particularly in recent decades, to work towards restoring Christian unity through ecumenical efforts.

Relations between the East and West have warmed over the last century, as both churches embrace a dialogue of charity. The Second Vatican Council (1962–1965) ushered in a new era of relations for the Roman Church towards the Orthodox Church, fondly describing the Orthodox as “separated brethren” with valid sacraments and an apostolic priesthood. The Orthodox Church, on the other hand, encouraged local churches to prepare for future dialogue in the Third Pan-Orthodox Conference in Rhodes (1964), and has since engaged in several ecumenical efforts with the Vatican. Significantly, in 1965 Pope Paul VI and Ecumenical Patriarch Athenagoras I of Constantinople jointly lifted the mutual excommunications of 1054. More recently, in November 2019, the current Patriarch of Constantinople, Patriarch Bartholomew, has stated he believes Orthodox re-union with the Catholic Church is inevitable.

Ecumenism (/ɪˈkjuːməˌnɪzəm/ ih-KEW-mə-niz-əm; alternatively spelled oecumenism) – also called interdenominationalism, or ecumenicalism – is the concept and principle that Christians who belong to different Christian denominations and theological traditions should work together to develop closer relationships among their churches and promote Christian unity. The adjective ecumenical is thus applied to any non-denominational or inter-denominational initiative which encourages greater cooperation and union among Christian denominations and churches. Ecumenical dialogue is a central feature of contemporary ecumenism.

The fact that all Christians belonging to mainstream Christian denominations profess faith in Jesus, believe that the Bible is inspired by God, 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 "may all be one" in order "that the world may know" and believe the Gospel message.