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Orthodox Christianity is a collective term for the Eastern Orthodox Church and Oriental Orthodoxy. These branches of Christianity use the term "orthdoxy" (from Greek: orthos + doxa, meaning correct belief) to express their belief to have an unbroken connection to the faith, doctrine and practices of the ancient Christian church. These two branches are the Eastern Orthodox Church and Oriental Orthodoxy. The adjectives "Eastern" and "Oriental" are used by outsiders to differentiate the two groups; their adherents call themselves simply "Orthodox Christians". The two groups have been divided by their disagreements over the nature of Christ since the 5th century, and they are currently not in communion with each other, but they maintain many identical doctrines, similar Church structures, and similar worship practices. There have been a number of recent talks aimed at reunification, and a great deal of agreement has been reached, but no concrete steps have been taken towards formal unity as yet.
Orthodox Churches in Slavic-language countries (Bulgaria, Russia, Serbia, Macedonia, etc.) use a word derived from Old Church Slavonic, Правосла́виѥ (pravosláviye) to mean orthodoxy. The word derives from the Slavonic roots "право" (právo, true, right) and "славить" (slávit, to praise, to glorify), in effect meaning "the right way to praise God".
Both the Eastern Orthodox Church and Oriental Orthodoxy use (with a small difference in plural/singular form of the verbs "we believe", "we confess", "we await") the original form of the Nicene Creed developed at the First Council of Constantinople in 381, referred to as the Niceno-Constantinopolitan Creed. In contrast, the Latin branch of the Roman Catholic Church and the Protestant churches of western Christianity add the phrases "God from God" and "and the Son" (see Filioque clause), and the Armenian Apostolic Church has many more additions. The addition of "and the Son" was (along with the Papal supremacy and some other questions) one of the causes for the East–West Schism formalized in 1054 by simultaneous proclamations of "Anathema" by the Bishop of Rome (Pope) in the West and the leadership of the Orthodox Churches (Patriarch) in the East.
Both Orthodox churches recognize Ecumenical councils as the highest authority on matters of faith, worship, and Church organization, but they disagree on the number of councils they consider to be Ecumenical. The Eastern Orthodox Church recognizes seven Ecumenical councils, while Oriental Orthodoxy recognizes only the first three.
Aphoristically said, "The Orthodox Church is evangelical, but not Protestant. It’s orthodox, but not Jewish. It’s catholic, but not Roman. It isn’t non-denominational – it’s pre-denominational. It has believed, taught, preserved, defended and died for the Faith of the Apostles since the day of Pentecost 2000 years ago." 
The Eastern and Oriental Orthodox Churches are both organized along similar lines. All bishops are considered to be sacramentally equal, no bishop holds any more spiritual authority than any other (thus, no single bishop is infallible), and a bishop cannot interfere with other dioceses that are not under his own jurisdiction. For the purpose of making practical administrative decisions regarding the day-to-day business of the Church - but not for the purpose of deciding matters of faith - the bishops in a large geographical area are gathered together in a Holy Synod, presided by a head bishop (who carries a special title that varies from place to place). In most cases, the area under the administration of a Holy Synod corresponds to the territory of a nation-state, but this is not always the case and it is a relatively recent development. Each major Holy Synod is autocephalous, meaning that it is independent of the other Holy Synods and does not answer to them except in cases of heresy. Thus, both the Eastern Orthodox Church and Oriental Orthodoxy are organized according to a federal structure, and do not have any central leadership or any single human leader (unlike the Catholic Church). This is one of the distinctive features of Orthodox Christianity, to such a degree that some dictionaries define an "Orthodox Church" as a Christian organization "not accepting the authority of the Pope of Rome, and using elaborate and archaic forms of service." 
Although both the Eastern Orthodox and the Oriental Orthodox are decentralized for administrative purposes, they take different approaches to liturgical diversity. The Eastern Orthodox exhibit great uniformity in their worship, with Eastern Orthodox churches in different countries being identical in most respects except for the language they use. The Oriental Orthodox in different countries, on the other hand, use different liturgies, prayers, and other forms of worship. For this reason, Eastern Orthodoxy is called a Church, but Oriental Orthodoxy is called a family of Churches.
The legalism and political power that has shaped Catholicism, as well as the total authority of the Pope, contrasts with the Orthodox Church, who keep the doctrine that their Faith is "not of this world."
The Orthodox believe that among the original five Patriarchs and ancient Patriarchates (i.e., Rome, Constantinople, Alexandria, Antioch, and Jerusalem), a "primacy of honor", not of supremacy, waits for Rome. To use an Orthodox apostolic term, if the Pope were to return to the Church, he would become "first among equals". To disassociate the "See of Rome" from this "equalisation," Pope Benedict XVI recently dropped the title "Patriarch of the West," as he considered the designation as an attempt to Orientalize Western ecclesiology (or adopt Orthodox terms). Benedict still considers the five Sees, dating back to the first millennium, to be "Sister Churches within a certain ecumenical context.
The Roman Catholic Church believes that Papal Supremacy was set forth by Jesus, in Matthew 16:13-19:
'Who do men say that the Son of man is?' And they said, 'Some say John the Baptist, others say Elijah; and others Jeremiah or one of the prophets.' He said to them, 'But who do you say that I am?' Simon Peter replied, 'You are the Christ, the Son of the Living God.' and Jesus answered him, 'Blessed are you, Simon Bar-Jonah! For flesh and blood has not revealed this to you, but my Father Who is in Heaven. And I tell you, you are Peter, and on this rock I will build my Church, and the powers of death shall not prevail against it. I will give you the keys of the kingdom of heaven, and whatever you bind on earth shall be bound in heaven, and whatever you loose on earth shall be loosed in heaven.'
In contrast, the Orthodox understand Christ's words as referring to all apostles, and His promise as an inheritance to all who confess the same belief. More specifically, they understand Jesus to be addressing Peter's faith (his confession of belief) as being the Rock upon which His Church would be built. This is allegedly shown by the fact that the original Greek uses the feminine demonstrative pronoun when he says "upon this rock" (ταύτῃ τῇ πέτρᾳ); whereas, grammatically, if Jesus had been referring to Peter, he would have used the masculine. Therefore, in the Orthodox Church, Papal supremacy and infallibility are considered to be human inventions, rather than stemming from a declaration by Christ.
Orthodox Christianity originated in the Eastern Mediterranean, Asia Minor and the Middle East, particularly in those lands which used to belong to the Byzantine Empire, but going as far south as Ethiopia and as far northeast as Armenia. Today, Orthodox populations remain in the Middle East, but their numbers are relatively small. The Eastern Orthodox Church - the larger of the two Orthodox groups - has become the second largest Christian communion in the world (after Roman Catholicism), and it is mainly concentrated in Russia, Eastern Europe, and Greece. Significant numbers of believers also live in Syria and Lebanon. Meanwhile, Oriental Orthodoxy has remained the dominant religion of Ethiopia and Armenia, two of its ancient heartlands. Large numbers of Oriental Orthodox believers also live in Egypt and India.
The country with the greatest number of Eastern Orthodox Christians is Russia, and the country with the greatest number of Oriental Orthodox Christians is Ethiopia.
Some Greek Catholics in Ukraine, Romania, Lebanon, and Syria, worship according to the Byzantine rite, but follow the primacy of the Pope. In the 17th and 18th Centuries, changes such as nationalism and threats of attack forced many of these Eastern Catholic Churches to accept being led by the politically strong Roman Catholic Church, who promised them aid if they would convert. National rulers sympathetic to Catholicism often led this change. Most such churches follow liturgical practices identical to those of the Orthodox Church.
Today, the term "Western Orthodox" is applied to groups of Orthodox Christians in the United Kingdom, the United States and such western European countries as Denmark, Finland, France, Germany and the Netherlands. With the consent of their bishops, they use a Western rite, and often a Western calendar. This movement is similar to that of Eastern Catholicism, though it is smaller and more recent, and not organized along national lines.
The term Oriental Orthodoxy is used to refer to non-Chalcedonian eastern Christians. This is in contrast to members of the Eastern Orthodox Church, who accept the Council of Chalcedon (See Ecumenical Councils) and generally worship according to the Byzantine Rite. Those identified as Oriental Orthodox have traditionally been referred to as Monophysites. They are found in Egypt, Ethiopia, some parts of Syria, Iraq and Iran, Armenia, and southern India in Kerala State. They accept only the first three of the ecumenical councils.
Since the early twentieth century, some rapproachment has taken place between these and the Eastern Orthodox Churches, particularly in Syria. Dialogue among the groups' representatives has resulted in statements that their longstanding differences have been of phraseology all along, and a simple misunderstanding by each side of what each church represents. Eastern Orthodox leaders do not find this entirely satisfactory. In addition, they hold that it is not within any (national) Church's competence to use a General Holy Synod to bring about communion with a group, such as any of the Oriental Orthodox Churches, that had been anathematized by the Church as a whole. Such Eastern Orthodox Christians hold that it would take another Great and Holy Council of every Eastern Orthodox bishop together to reverse the Anathema. Needless to say, the convening of such a Council presents a considerable obstacle.
Since the Second Vatican Council (1962-5), changes in the Roman Catholic Church have led to a gradual "rapprochement" between Catholic Rome and Orthodoxy at the official level. The institutions' simultaneous revocations of the anathemas of 1054 were a gesture toward "restoring mutual trust" and a recognition that there is "a vast area of common ground that the two sides share." Regarding dogma, Orthodox often believe that
"Latin scholastic theology makes too much use of legal concepts, and relies too heavily on rational categories and syllogistic argumentation, while the Catholics for their part have frequently found the more mystical approach of Orthodoxy too vague and ill-defined." There are also "psychological barriers [in Eastern Europe] that need to be overcome."
For example, in 2008, Patriarch Alexi of All Russia complained about the presence of Catholic clerics and missionaries in Russia, noting, "If they consider Orthodoxy to have just as much the grace of God and salvation as Catholicism, then what is the point of persistent attempts to convert people to the other faith?"  The Russian Church, for example, in a gesture of good will, does not demand that Roman Catholics "receive Chrismation" when they convert to Orthodoxy, but allows a simple profession of faith ("though Anglican and other Protestants are always received by Chrismation").
The longstanding Christian institutions described above consider developments in Christianity since the nineteenth century to be unorthodox in the strongest possible sense (or cults). They include within that category Unitarians, the Church of Jesus Christ of Latter-day Saints (Mormons), Jehovah's Witnesses, and some of the more radical forms of Liberal Christianity.
Each of the major ecclesiastical communities continues to evaluate and grapple with issues that it believes are estrangements or refinements of perceived orthodoxy. For example, the Roman See often issues recommendations as to what worship practices it considers orthodox so as to curb excesses or deficiencies by its prelates. Some evangelicals are pursuing innovations that more conservative evangelicals consider unorthodox. The latter refer to the changes as "neo-evangelical", "neo-pentecostal," or "fringe Charismatic."
Difference between Eastern Orthodoxy and Oriental Orthodoxy 
Both branches of Orthodox Christianity, together with what is now the Roman Catholic Church, were united and single until the Council of Chalcedon in AD 451, after which they were divided by their theological interpretations of the Christ (dyophysite versus miaphysite). The Council of Chalcedon decided that Jesus exists "in two natures", one human and one divine, and that "both natures concur in one person and in one reality [hypostasis]. They are not divided or cut into two persons, but are together the one and only and only-begotten Word, God, the Lord Jesus Christ." The Council of Chalcedon thus declared that Christ is one person in two natures "of one substance with the Father according to his divinity, of one substance with us according to his humanity ... in two natures without confusion, without change, without division, without separation." This represents the dyophysite view of the Christ.
The majority of the world's Christians at the time - specifically, the patriarchates of Constantinople, Rome, Jerusalem, and parts of the patriarchates of Antioch and Alexandria - accepted the Council of Chalcedon as Ecumenical, and therefore also accepted its decision to endorse the dyophysite view of the Christ as part of Christian doctrine. These are known as Chalcedonian Christians, and they are the origin of today's Eastern Orthodox and Roman Catholic Churches. The split between Eastern Orthodox and Roman Catholics occurred much later, in the 11th century, but it is widely regarded by the Eastern Orthodox as representing a more severe break than the one resulting from Chalcedon (thus, Eastern Orthodox Christians consider themselves much closer to Oriental Orthodoxy than to Roman Catholicism).
Over a number of decades following AD 451, a significant minority of the world's Christians refused to accept the decisions of the Council of Chalcedon, affirming instead the miaphysite view of the Christ. They are known as non-Chalcedonian or pre-Chalcedonian Christians. They represented most of the faithful of the patriarchate of Alexandria (Egypt and Ethiopia), a large part of the patriarchate of Antioch (Syria), and the entire Church in Armenia. The Christians in India at the time also embraced the miaphysite view when they became aware of the controversy. Collectively, the non-Chalcedonians came to be known as Oriental Orthodox Churches.
Dialogues aimed at achieving full communion between Eastern and Oriental Orthodox are in progress, with the hope of overcoming the schism that still divides them. These dialogues have led to a large measure of agreement, but not yet to full normal communion.
The Eastern Orthodox Church uses the Byzantine Rite liturgy almost exclusively, celebrating it in different languages. The only exceptions to this are a small number of Western Rite parishes, which have adapted or specially composed liturgies based on Latin liturgical rites. The Oriental Orthodox Churches, on the contrary, use a great variety of liturgies.
Some churches of the Eastern Orthodox tradition are not in communion with the general body, usually because of disputes about the use of the Julian calendar, but in some cases because of political problems. There is also one such case in Oriental Orthodoxy, namely that of the Malabar Independent Syrian Church in India.
- "The Oriental Orthodox churches, along with those of the Byzantine tradition or Eastern Orthodox, belong to the larger family of the Orthodox churches. The two groups are not in communion with each other" (Orthodox churches (Oriental)
- Archimandrite Nektarios Serfes. What Is The Orthodox Church
- Nicene Creed (Armenian Apostolic Church); The Coptic Orthodox Church: Our Creed (Coptic Orthodox Church of Alexandria); Nicene Creed (Ethiopian Orthodox Tewahedo Church); The Nicene Creed (Malankara Orthodox Syrian Church); The Nicene Creed (Syriac Orthodox Church).
- Text in Armenian, with transliteration and English translation
- "Orthodox Church". Orthodox Church Website.
- "'Our Life in Christ' Orthodox Radio Program Site". Our Life in Christ.
- "Definition of Orthodox Church". Oxford dictionaries online. Retrieved November 27, 2012.
- Timothy Wise, Orthodoxy (London: Penguin Books, 1993), 27.
- John Allen, "The 'Patriarch of the West' Retires," The National Catholic Reporter, 7 April 2006, 21.
- Joseph Ratzinger, "Sister Churches," The Tablet, 9 September 2000, 1205
- Holy Apostles Convent (1999) The Orthodox New Testament, Vol. I: The Holy Gospels (Dormition Skete, Buena Vista CO, ISBN 0-944359-13-2) p. 105
- Timothy Ware, The Orthodox Church (London: Penguin, 1993), 314-315.
- Ronald Roberson, "Russian Steps to unity," The Tablet, 13 December 2008, 15.
- Ware (1993), 279.
- Definition concerning the two natures of Christ
- J. Martin Bailey. Who are the Christians in the Middle East? (Eerdmans 2003), p. 66
- Ronald G. Roberson, The Eastern Christian Churches: a brief survey, p. 5
- James Leslie Houlden, Jesus in History, Thought and Culture: An Encyclopedia, volume 1, p. 666
- Thomas E. Fitzgerald, The Ecumenical Movement: An Introductory History, p. 56]
- Pastoral Agreement between the Coptic Orthodox and Greek Orthodox Patriarchates of Alexandria (2001)
- Frequently Asked Questions About the Oriental Orthodox
- The Coptic Orthodox Church
- Growth in Agreement II, by Jeffrey Gros, Harding Meyer, William G. Rusch (Eerdmans Publishing, 2000 ISBN 978-2-8254-1329-6) chapter VIII: Eastern Orthodox-Oriental Orthodox Dialogue
- Texts of the Agreed Statements of the Joint Commission
- "The Orthodox all the world over must follow the Rite of Constantinople. In this unjustifiable centralization we have a defiance of the old principle, since Antioch, Jerusalem, Alexandria, Cyprus, in no way belong to the Byzantine Patriarchate" (Adrian Fortescue, Rites).
- "There are three Ethiopic liturgies all addressed to the Son. In Syria itself the Monophysite Second Liturgy of S. Peter ... (is) directed to the Son, as is part of the eucharistic prayer of the Syriac St James itself, which is followed in this by nearly all the sixty or seventy lesser Syriac liturgies" (The Shape of the Liturgy, by Gregory Dix, published by Continuum, 2005, page 180). The Coptic Church uses the Liturgy of St Cyril, the Liturgy of St Gregory the Theologian, and the Coptic Liturgy of St Basil. In Ethiopia, the most commonly used liturgy is the Ethiopian Orthodox Liturgy of the Apostles.