Eucharist in the context of "Roman Mass"

The Eastern Orthodox Church, officially the Orthodox Catholic Church, and also called the Greek Orthodox Church or simply the Orthodox Church, is one of the three major doctrinal and jurisdictional groups of Christianity. As of 2012, it has approximately 300 million adherents and is the third largest religious community in the world after Catholics and Sunni Muslims. The Eastern Orthodox Church operates as a communion of autocephalous churches, each governed by its bishops via local synods. The church has no central doctrinal or governmental authority analogous to the pope of the Catholic Church. Nevertheless, the Ecumenical Patriarch of Constantinople is recognised by them as primus inter pares ('first among equals'), a title held by the patriarch of Rome prior to 1054. As one of the oldest surviving religious institutions in the world, the Eastern Orthodox Church has played an especially prominent role in the history and culture of Eastern and Southeastern Europe. Since 2018, there has been an ongoing schism between Constantinople and Moscow, with the two not in full communion with each other.

Eastern Orthodox theology is based on the Scriptures and holy tradition, which incorporates the dogmatic decrees of the seven ecumenical councils, and the teaching of the Church Fathers. The church teaches that it is the one, holy, catholic and apostolic church established by Jesus Christ in his Great Commission, and that its bishops are the successors of Christ's apostles. It maintains that it practises the original Christian faith, as passed down by holy tradition. Its patriarchates, descending from the pentarchy, and other autocephalous and autonomous churches, reflect a variety of hierarchical organisation. It recognises seven major sacraments (which are called holy mysteries), of which the Eucharist is the principal one, celebrated liturgically in synaxis. The church teaches that through consecration invoked by a priest, the sacrificial bread and wine become the body and blood of Christ. The Virgin Mary is venerated in the Eastern Orthodox Church as the Theotokos, which means 'God-bearer', and she is honoured in devotions.

Full communion is a communion or relationship of full agreement among different Christian denominations or Christian individuals that share certain essential principles of Christian theology. Views vary among denominations on exactly what constitutes full communion, but typically when two or more denominations are in full communion it enables services and celebrations, such as the Eucharist, to be shared among congregants or clergy of any of them with the full approval of each.

A sacrament is a Christian rite which is recognized as being particularly important and significant. There are various views on the existence, number and meaning of such rites. Many Christians consider the sacraments to be a visible symbol of the reality of God, as well as a channel for God's grace. Many denominations, including the Catholic, Lutheran, Moravian, Reformed (Continental Reformed, Presbyterian, and Congregationalist traditions), Anglican, Methodist and Baptist, hold to the definition of sacrament formulated by Augustine of Hippo: an outward sign of an inward grace, that has been instituted by Jesus Christ. Sacraments signify God's grace in a way that is outwardly observable to the participant.

The Catholic Church, Hussite Church and the Old Catholic Church recognize seven sacraments: Baptism, Penance (Reconciliation or Confession), Eucharist (or Holy Communion), Confirmation, Marriage (Matrimony), Holy Orders, and Anointing of the Sick (Extreme Unction). The Eastern Churches, such as the Eastern Orthodox Church and Oriental Orthodox Church as well as the Eastern Catholic Churches, recognize these as the seven major sacraments, but also apply the words sacred mysteries corresponding to Greek word, μυστήριον (mysterion), and also to rites that in the Western tradition are called sacramentals and to other realities, such as the Church itself. Many Protestant denominations, such as those within the Reformed tradition, preach just two sacraments instituted by Christ: the Eucharist (or Holy Communion) and Baptism. The Lutheran sacraments include these two, often adding Confession (and Absolution) as a third sacrament. Anglican and Methodist teaching is that "there are two Sacraments ordained of Christ our Lord in the Gospel, that is to say, Baptism and the Supper of the Lord", and that "those five commonly called Sacraments, that is to say, Confirmation, Penance, Orders, Matrimony, and Extreme Unction, are not to be counted for Sacraments of the Gospel."

Divine Liturgy (Koine Greek: Θεία Λειτουργία, romanized: Theía Leitourgía, pronounced [ˈθi.a li.turˈʝi.a]) or Holy Liturgy is the usual name used in most Eastern Christian rites for the Eucharistic service.

The Eastern Orthodox Churches, Eastern Catholic Churches and Eastern Lutheran Churches believe the Divine Liturgy transcends both time and the world. All believers are seen as united in worship in the Kingdom of God along with the departed saints and the angels of heaven. Everything in the liturgy is seen as symbolic, but not merely so, for it makes present the unseen reality. According to Eastern tradition and belief, the liturgy's roots go back to the adaptation of Jewish liturgy by Early Christians. The first part, termed the "Liturgy of the Catechumens", includes the reading of scriptures like those in a synagogue, and in some places, also a sermon/homily. The second half, the "Liturgy of the Faithful", is based on the Last Supper and the first Eucharistic celebrations by Early Christians. Eastern Christians (and many other branches of Christianity) believe that the Eucharist is the central part of the service in which they participate, as they believe the bread and wine truly become the real Body and Blood of Christ, and that by partaking of it they jointly become the Body of Christ (that is, the Church). Each liturgy has its differences from others, but most are very similar to each other with adaptations based on tradition, purpose, culture and theology.

The East–West Schism, also known as the Great Schism or the Schism of 1054, is the break of communion between the Catholic Church and the Eastern Orthodox Church since 1054. A series of ecclesiastical differences, theological disputes and geopolitical tensions between the Greek East and Latin West preceded the formal split that occurred in 1054. Prominent among these were the procession of the Holy Spirit (Filioque), whether leavened or unleavened bread should be used in the Eucharist, iconoclasm, the coronation of Charlemagne as emperor of the Romans in 800, the pope's claim to universal jurisdiction, and the place of the See of Constantinople in relation to the pentarchy.

The first action that led to a formal schism occurred in 1053 when Patriarch Michael I Cerularius of Constantinople ordered the closure of all Latin churches in Constantinople. In 1054, the papal legate sent by Leo IX travelled to Constantinople in order, among other things, to deny Cerularius the title of "ecumenical patriarch" and insist that he recognize the pope's claim to be the head of all of the churches. The main purposes of the papal legation were to seek help from the Byzantine emperor, Constantine IX Monomachos, in view of the Norman conquest of southern Italy, and to respond to Leo of Ohrid's attacks on the use of unleavened bread and other Western customs, attacks that had the support of Cerularius. The historian Axel Bayer says that the legation was sent in response to two letters, one from the emperor seeking help to organize a joint military campaign by the eastern and western empires against the Normans, and the other from Cerularius. When the leader of the legation, Cardinal Humbert of Silva Candida, O.S.B., learned that Cerularius had refused to accept the demand, he excommunicated him, and in response Cerularius excommunicated Humbert and the other legates. According to Kallistos Ware, "Even after 1054 friendly relations between East and West continued. The two parts of Christendom were not yet conscious of a great gulf of separation between them ... The dispute remained something of which ordinary Christians in East and West were largely unaware".

The Eastern Catholic Churches or Oriental Catholic Churches, also known as the Eastern-Rite Catholic Churches, Eastern Rite Catholicism, or simply the Eastern Churches, are 23 Eastern Christian autonomous (sui iuris) particular churches of the Catholic Church in full communion with the pope in Rome. Although they are distinct theologically, liturgically, and historically from the Latin Church, they are all in full communion with it and with each other. Eastern Catholics are a minority within the Catholic Church; of the 1.3 billion Catholics in communion with the pope, approximately 18 million are members of the Eastern churches. The largest numbers of Eastern Catholics are found in Eastern Europe, Eastern Africa, the Middle East, and India. As of 2022, the Syro-Malabar Church is the largest Eastern Catholic Church, followed by the Ukrainian Greek Catholic Church.

With the exception of the Maronite Church, the Eastern Catholic Churches are groups that, at different points in the past, used to belong to the Eastern Orthodox Church, the Oriental Orthodox churches, or the Church of the East; these churches underwent various schisms through history. Eastern Catholic Churches that were formerly part of other communions have been points of controversy in ecumenical relations with the Eastern Orthodox and other non-Catholic churches. The five historic liturgical traditions of Eastern Christianity, namely the Alexandrian Rite, the Armenian Rite, the Byzantine Rite, the East Syriac Rite, and the West Syriac Rite, are all represented within Eastern Catholic liturgy. On occasion, this leads to a conflation of the liturgical word "rite" and the institutional word "church". Some Eastern Catholic jurisdictions admit members of churches not in communion with Rome to the Eucharist and the other sacraments.

The Eastern Catholic Churches of the Catholic Church utilize liturgies originating in Eastern Christianity, distinguishing them from the majority of Catholic liturgies which are celebrated according to the Latin liturgical rites of the Latin Church. While some of these sui iuris churches use the same liturgical ritual families as other Eastern Catholic churches and Eastern churches not in full communion with Rome, each church retains the right to institute its own canonical norms, liturgical books, and practices for the ritual celebration of the Eucharist, other sacraments, and canonical hours.

Historically, tension between Latin Catholics and those worshipping with Eastern liturgies resulted in the latinization, restriction, or prohibition of Eastern liturgies within the Catholic Church. Since the early 20th century, popes have encouraged the usage of traditional liturgies among Eastern Catholics and delatinization. Further emphasis on Eastern Catholic liturgical practice was made during the Second Vatican Council with the publication of the 1964 Orientalium Ecclesiarum.

Liturgy is the customary public ritual of worship performed by a religious group. As a religious phenomenon, liturgy represents a communal response to and participation in the sacred through activities reflecting praise, thanksgiving, remembrance, supplication, or repentance. It forms a basis for establishing a relationship with God or the divine.

Technically speaking, liturgy forms a subset of ritual. The word liturgy, sometimes equated in English as "service", refers to a formal ritual enacted by those who understand themselves to be participating in an action with the divine.

Reformed Christianity, also called Calvinism, is a major branch of Protestantism that began during the 16th-century Protestant Reformation. In the modern day, it is largely represented by the Continental Reformed, Presbyterian, and Congregational traditions, as well as parts of the Anglican (known as "Episcopal" in some regions), Baptist and Waldensian traditions, in addition to a minority of persons belonging to the Methodist faith (who are known as Calvinistic Methodists).

Reformed theology emphasizes the authority of the Bible and the sovereignty of God, as well as covenant theology, a framework for understanding the Bible based on God's covenants with people. Reformed churches emphasize simplicity in worship. Several forms of ecclesiastical polity are exercised by Reformed churches, including presbyterian, congregational, and some episcopal. Articulated by John Calvin, the Reformed faith holds to a spiritual (pneumatic) presence of Christ in the Lord's Supper.