The Catholic Church has engaged in the modern ecumenical movement especially since the Second Vatican Council (1962–1965) and the issuing of the decree Unitatis redintegratio and the declaration Dignitatis humanae. It was at the Council that the Pontifical Council for Promoting Christian Unity was created. Those outside of the Catholic Church were categorised as heretics (in reference to Protestantism) or schismatics (as in the case of the Eastern Orthodox Church and Oriental Orthodox Churches), but in many contexts today, to avoid offence, the euphemism "separated brethren" is used.

Since the mid-20th century, the Catholic Church in Unitatis redintegratio has emphasized the common Trintarian baptism of Catholic, Protestant, and Orthodox Christians. The Directory for the Application of Principles and Norms on Ecumenism states that "In liturgical celebrations taking place in other churches and ecclesial communities, Catholics are encouraged to take part in the psalms, responses, hymns, and common actions of the church in which they are guests."

Definition

Ecumenism, from the Greek word "oikoumene", meaning "the whole inhabited world" (cf. Acts 17.6; Mt 24.14; Heb 2.5), is the promotion of cooperation and unity among Christians. The Union of Christendom is a traditional Catholic view of ecumenism; the view is that every non-Catholic Christian ecclesial community is destined to return to the unity of the Catholic Church, from which it has broken. As the original Church founded by Jesus Christ according to Catholic doctrine, the Church sees itself as "the one true church". This means that according to such teaching, Christian unity is already a reality, present in the Catholic Church.

The Catholic Church's commitment to ecumenism was based on the conviction that a divided Christianity "openly contradicts the will of Christ, scandalizes the world, and damages the holy cause of preaching the Gospel to every creature".

"The search for Christian Unity was one of the principal concerns of the Second Vatican Council". This was attested to even from the moment the council was announced, by Pope John XXIII, during the vespers closing the Octave of Prayer for Christian Unity on 25 January 1959, at the Basilica of Saint Paul Outside the Walls.

"The Catholic Church is committed to working for the reunion of all Christians, but the exuberant spirit following Vatican II has been tempered. Sober minds realize that the road to full unity will be long and arduous. One of the principal ecclesiological tasks is to discern the relationship between the Churches." Underlying the Catholic Church's pursuit of ecumenism is its recognition that elements of sanctification and truth are found in other churches, that, in some sense, these are Christians and Particular Churches or ecclesial communities, and that common baptism itself impels those toward greater unity, since Baptism is a gift which "belongs" to the Catholic Church as one of the Sacraments which Christ endowed it with.

It can be said that the "ecumenicity" of the Church is another way of expressing her "radical catholicity and/or universality".Guidelines for Lutheran-Roman Catholic Dialogue 1967

The Directory for the Application of Principles and Norms on Ecumenism states that bishops of the Catholic Church, "individually for their own dioceses, and collegially for the whole Church, are, under the authority of the Holy See, responsible for ecumenical policy and practice".

Historical perspectives Before the Second Vatican Council The Catholic Church sees itself as the one, holy, catholic and apostolic church, founded by Christ himself. Its teachings, both before and after the Second Vatican Council, equate the one Church of Christ with the Catholic Church.

Ecumenism takes as it starting point that Christ founded just one Church, not many churches; hence the Catholic Church has as its ultimate hope and objective that through prayer, study, and dialogue, the historically separated bodies may come again to be reunited with it.

Unity was always a principal aim of the Catholic Church. Before the Second Vatican Council, the Catholic Church defined ecumenism as dialogue with other Christian groups to persuade these to return to a unity that they themselves had broken.

At the Second Council of Lyon (1274) and the Council of Florence (1438–42), in which some bishops of the Eastern Orthodox Churches participated, reunion formulas were worked out, but these failed to win acceptance by the Eastern Churches.

The Catholic Church, even before the Second Vatican Council, always considered it a duty of the highest rank to seek full unity with estranged communions of fellow-Christians, and at the same time to reject what it saw as premature and false union that would mean being unfaithful to or glossing over the teaching of Sacred Scripture and Tradition. But the main stress was laid on caution, as exemplified in canon 1258 the 1917 Code of Canon Law: It is illicit for the faithful to assist at or participate in any way in non-Catholic religious functions. For a serious reason requiring, in case of doubt, the Bishop's approval, passive or merely material presence at non-Catholic funerals, weddings and similar occasions because of holding a civil office or as a courtesy can be tolerated, provided there is no danger of perversion or scandal.

Second Vatican Council The aim of the Second Vatican Council, as its initiator Pope John XXIII stated, was to seek renewal from within the Church itself, which would serve for those separated from the See of Rome as a "gentle invitation to seek and find that unity for which Jesus Christ prayed so ardently to his heavenly Father".Pope John XXIII, Ad Petri cathedram The Council opened up an era of earnest endeavour not only to explain to others the Church's teaching, but also to understand their outlook.

While the Catholic Church sees itself as "the one, holy, catholic and apostolic church" founded by Christ himself, it recognizes that:

The Council's decree Unitatis redintegratio (1964) was exclusively devoted to ecumenism, confirming that "the restoration of unity among all Christians is one of [its] principal concerns".

In its Decree on the Apostolate of the Laity (1965) the Council also stated that:

Later developments

Since the Second Vatican Council, under Pope Paul VI, and the beginning of the papacy of John Paul II, the Catholic Church has reached out to other Christian bodies, seeking reconciliation to the greatest degree possible.

Significant agreements have been achieved on baptism, ministry, and the Eucharist with Anglican theologians. With Evangelical Lutheran bodies, a similar agreement was reached on the theology of justification. These landmark documents have brought closer fraternal ties with those churches.

However, recent developments, such as the ordination of women and of men living in homosexual relationships, present new obstacles to reconciliation with, in particular, Anglicans/Episcopalians. Consequently, in recent years the Catholic Church has focused its efforts on reconciliation with the Eastern Orthodox Church, with which the theological differences are not as great.

While relations with some Eastern Orthodox Churches were strained in the 1990s over property issues in post-Soviet states after the dissolution of the Soviet Union in 1989–1991, these differences are now largely resolved. Fraternal relations with the Eastern churches continue to progress.

The 1983 Code of Canon Law no longer has canons which absolutely forbid the social cooperation of Roman Catholic clergy (bishops, priests and deacons) with clergy members of other systems of belief. It still absolutely forbids Catholic priests to concelebrate the Eucharist with members of communities not in communion with the Catholic Church (canon 908). The Directory for the Application of Principles and Norms on Ecumenism states that "Christians may be encouraged to share in spiritual activities and resources, i.e., to share that spiritual heritage they have in common in a manner and to a degree appropriate to their present divided state."

Relations with the World Council of Churches One of the most significant documents on ecumenical relations was Baptism, Eucharist and Ministry, published by the Faith and Order Commission of the World Council of Churches (WCC) in 1982.

Although the Catholic Church is not a member of the WCC, since 1968 some Catholic theologians have been full members of the commission. Although not as representatives of their church, Catholic theologians participated in the production of the 1982 paper, with the aim to seek common ground between the various traditions concerning the Christian rite of initiation (Baptism), the sacrament of the Eucharist, and the nature of holy orders, while also stating clearly the differences existing between them. The churches were invited to indicate their reactions to the contents of the document, with a view to "analyz(ing) the ecumenical implications for the churches at a future World Conference on Faith and Order."

Relations with churches of the East The Catholic Church recognizes 21 Ecumenical or General Councils: Nicaea I (325), Constantinople I (381), Ephesus (431), Chalcedon (451), Constantinople II (553), Constantinople III (680–681), Nicaea II (787), Constantinople IV (869–870), Lateran I (1123), Lateran II (1139), Lateran III (1179), Lateran IV (1215), Lyons I (1245), Lyons II (1274), Vienne (1311–1312), Constance (1414–1418), Florence (1438–1445), Lateran V (1512–1517), Trent (1545–1563), Vatican I (1869–1870), Vatican II (1962–1965).

Of these, the Eastern Orthodox Church of Byzantine tradition accepts only the first seven, the family of "non-Chalcedonian" or "pre-Chalcedonian" Churches only the first three, and the Assyrian Church of the East only the first two.

In spite of this, dialogue has shown that even where divisions have lasted longer than previous communion – as with the schisms rooted in the Christological controversies at the Council of Ephesus (431) and the Council of Chalcedon (451) – the few doctrinal differences often but not always concern terminology, not substance. In fact, as little as a decade of dialogue has done more to heal the divisions than fifteen centuries of apologetics, stereotypes, and suspicion.

Assyrian Church of the East

The Catholic Church recognizes in the Assyrian Church of the East as one of the valid successor bodies of the ancient Church of the Mesopotamian valley, where Christianity had been established by around the year 150, though tradition traces apostolic origins with the Apostle Thomas and his disciples-successors Addai and Mari.

The division between the Church of the Persian Empire and the Churches of Rome and Constantinople goes back to the disputes over the legitimacy of the terms mother of God and mother of Christ for the Virgin Mary, that came to a head at the Council of Ephesus in 431. The Assyrian Church had adopted radical Antiochene Christology, as articulated by Nestorius and Theodore of Mopsuestia. They have been sometimes, erroneously, called Nestorian. This appellation is rejected by the Catholic Church.

Dialogue began with a meeting of the Assyrian Patriarch (Catholicos) Mar Dinkha IV and Pope John Paul II in 1984, and the patriarch's participation in the first Assisi Day of Prayer for Peace in 1986. Nearly a decade of dialogue proved sufficient to resolve the disagreements over terminology in Christology, leading to the Common Christological Declaration Between the Catholic Church and the Assyrian Church of the East, signed by Pope John Paul II of the Catholic Church and Mar Dinkha IV of the Assyrian Church of the East in 1994.