Eastern Christians have a shared tradition, but they became divided (schism) during the early centuries of Christianity in disputes about christology and fundamental theology.
In general terms, Eastern Christianity can be described as comprising four families of churches: the Assyrian Church of the East, the Eastern Orthodox Churches, Oriental Orthodoxy, and the Eastern Catholic Churches.
Although there are important theological and dogmatic disagreements among these groups, nonetheless in some matters of traditional practice that are not matters of dogma, they resemble each other in some ways in which they differ from Catholic and Protestant churches in the West. For example, in all the Eastern churches, parish priests administer the sacrament of chrismation to newborn infants just after baptism; that is not done in Western churches. All the groups have weaker rules on clerical celibacy than those of the Latin Rite (i.e., Western) Catholic churches, in that, although they don't allow marriage after ordination, they allow married men to become priests (and originally bishops). For these reasons, it sometimes makes sense to generalize, saying "In the Eastern Church, it is customary to ..." etc.
The Eastern churches' differences from Western Christianity have as much, if not more, to do with culture, language, and politics as theology. For the non-Catholic Eastern churches, a definitive date for the commencement of schism cannot be given (see East-West Schism), although conventionally, it is often stated that the Assyrian Church of the East became estranged from the church of the Roman Empire in the years following the Council of Ephesus (431), Oriental Orthodoxy separated after the Council of Chalcedon (451), and the split between the Church of Rome and the Orthodox Church is usually dated to 1054 (often referred to as the Great Schism).
Oriental Orthodoxy developed in reaction to Chalcedon on the eastern limit of the Byzantine Empire and in Egypt and Syria. In those locations, there are now also Eastern Orthodox Patriarchs, but the rivalry between the two has largely vanished in the centuries since schism.
The following Oriental Orthodox churches are autocephalous and in full communion:
Orthodox Christianity identifies itself as the original Christian church founded by Christ and the Apostles, and traces its lineage back to the early church through the process of Apostolic Succession and unchanged theology and practice. Orthodox distinctives (shared with some of the Eastern Catholic Churches) include the Divine Liturgy, Mysteries or Sacraments, and an emphasis on the preservation of Tradition, which it holds to be Apostolic in nature.
Orthodox Churches are also distinctive in that they are organized into selfgoverning jurisdictions along national lines. Orthodoxy is thus made up of 14 or 15 national autocephalous bodies. Smaller churches are autonomy and each have a mother church that is autocephalous.
The Eastern Orthodox Church includes the following churches
Most Eastern Orthodox are united in communion with the Ecumenical Patriarch of Constantinople, though unlike in the Roman Catholic Church, this is a looser connection rather than a top-down hierarchy (see primus inter pares).
It may also be noted that the Church of Rome was once in communion with the Eastern Orthodox Church, but the two were split after the East-West Schism and thus it is no longer in communion with the Eastern Orthodox Church.
It is estimated that there are approximately 240 million Orthodox Christians in the world. Today, many adherents shun the term "Eastern" as denying the church's universal character. They refer to Eastern Orthodoxy simply as the Orthodox Church.
The twenty-two Eastern Catholic churches are all in communion with the Holy See at the Vatican, but are rooted in the theological and liturgical traditions of Eastern Christianity.
Many of these churches were originally part of one of the above families and so are closely related to them by way of ethos and liturgical practice. As in the other Eastern churches, married men may become priests, and parish priests administer the mystery of confirmation to newborn infants immediately after baptism, via the rite of chrismation; the infants are then administered Holy Communion.
The Maronite Church always remained in communion with the Holy See, and thus does not have a counterpart among the non-Catholic Eastern churches. The (Italo-Albanian) Italo-Greek Catholic Church has come under Papal authority very soon under the Schism, and thus has no non-popist counterpart. Eastern Catholics form around 2% of the entire membership of the Catholic Church. Most of the Eastern Catholic churches re-established communion with Rome during the 17th through 19th centuries.
At the same time, the Commission stated:
There are national dissidents, where ethnic groups want their own nation-church like with the Macedonian Orthodox Church and Montenegrin Orthodox Church; both domiciles of the Serbian Orthodox Church. However, it should be noted that in Macedonia, the influence of the Serbian Orthodox Church is minimal to non-existent. The vast majority of Orthodox ethnic Macedonians view the Serbian Orthodox Church as hostile to Macedonian history, national interests, and self-determination.
A little known movement of "reformers" in the Greek Orthodox Church traces its history to the 18th century. The leaders of this "schism" within the Orthodox Christian churches were called by a Greek word meaning 'unstable' (astateos). The children of these leaders left the East toward Western Europe, mainly Spain. In Ibero America these families are known by the derivative name 'Astacios' or 'Astacio.' One of their descendants was one of the first converts to the Pentecostal movement in 1916, Petra Astacio, of Montellano (Ponce, Puerto Rico). The Astacios have intermarried with native people of the Americas as well as with Spanish Jews (Sephardim) and Afro-Caribbeans.
The Eastern churches (excepting the non-liturgical dissenting bodies) each belong to one of several liturgical families:
For other definitions and meaning for the word orthodox, see Orthodoxy.