The word orthodox, from Greek orthodoxos "having the right opinion," from orthos ("right, true, straight") + doxa ("opinion, praise", related to dokein, "thinking"), is typically used to mean adhering to the accepted or traditional and established faith, especially in religion.
The term did not conventionally exist with any degree of formality (in the sense in which it is now used) prior to the advent of Christianity in the Greek-speaking world, though the word does occasionally show up in ancient literature in other, somewhat similar contexts. Orthodoxy is opposed to heterodoxy ("other teaching"), heresy and schism. People who deviate from orthodoxy by professing a doctrine considered to be false are most often called heretics or radicals, while those who deviate from orthodoxy by removing themselves from the perceived body of believers are called schismatics. The distinction in terminology pertains to the subject matter; if one is addressing corporate unity, the emphasis may be on schism; if one is addressing doctrinal coherence, the emphasis may be on heresy.
Apostasy, for example, is a violation of orthodoxy that takes the form of abandonment of the faith, a concept largely unknown before the adoption of Christianity as the state religion of Rome on February 27, 380 by Theodosius I, see also First seven Ecumenical Councils. A lighter deviation from orthodoxy than heresy is commonly called error, in the sense of not being grave enough to cause total estrangement, while yet seriously affecting communion. Sometimes error is also used to cover both full heresies and minor errors.
The concept of orthodoxy is the most prevalent and even inherently pervasive in nearly all forms of organized monotheism, but orthodox belief is not usually overly emphasized in polytheistic or animist religions. Often there is little to no concept of dogma, and varied interpretation of doctrine and theology is tolerated and sometimes even encouraged within certain contexts. Syncretism, for example, plays a much wider role in non-monotheistic (and particularly, non-scriptual) religion. The prevailing governing idea within polytheism is most often orthopraxy ("right practice") rather than "right belief".
The Eastern Orthodox Churches uses the original form of the Nicene Creed created at the First Council of Constantinople in 381, in contrast to the Roman Catholic church, which uses the Nicene creed with the addition of the phrase 'and the Son' (see Filioque clause). This change is one of many causes for the Great Schism formalized in 1054 by simultaneous proclamations of "Anathema" from the leadership of the Orthodox Churches in the East and the Bishop of Rome (Pope) in the West. This emphasis on the use of the original "creed" is shared today by all Eastern Orthodox churches.
The orthodox church has relations with the Catholic church but also has many differences. The Roman Catholic Church considers the Eastern Orthodox to be in schism and not in full communion with the Holy See. But the Roman Catholic Church does not consider the Eastern Orthodox church to be schismatic and heretical. Although the Roman Catholic Church recognizes that the Eastern Orthodox church has valid sacraments and full apostolic succession many Roman Catholics do not even know that Eastern Orthodoxy exists. Recent declarations between the two churches in recent years have brought the two churches closer together than they had been for centuries. A joint commission of Orthodox and Catholic theologians agreed that the Pope has primacy over all bishops, though disagreements about the extent of his authority still continue, see also Papal primacy. The Joint Commission for Theological Dialogue reached the agreement in a meeting in Ravenna, Italy in October 2006.
Confusingly, the term "Western Orthodox" is used to refer to Uniate Catholic churches in communion with the Roman See, also known as Eastern Catholic Churches. Today "Western Orthodox" will probably refer to groups of apostolic Orthodox Christians in the United Kingdom, USA, and perhaps smaller numbers in Denmark, Finland, France, Germany and the Netherlands, who wish to be Orthodox and yet want a western and Latin rite. It can also refer to the Orthodox churches that have implemented a Western rite such as the Antiochian Orthodox church.
In Ukraine and Romania there are Uniates called Greek Catholics who have the Byzantine rite, but accept the primacy of the Pope, and so are called Byzantine Catholics. Also, in Lebanon and Syria are groups called Maronites and Melkites in a similar situation. Their numbers are relatively small when compared to the size of the Orthodox Churches – though the Melkite church numbers over a million faithful.
The term Oriental Orthodoxy is used to refer to non-Chalcedonian eastern Christians, as opposed to Christians of Eastern Orthodox Churches, who accept the Council of Chalcedon (See Ecumenical Councils) and generally worship according to the Byzantine Rite. They have been traditionally referred to as Monophysite. 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. In the last century there has been some rapproachement between these and the Eastern Orthodox Churches, particularly in Syria. There have been claims after dialogue, that really the differences have been of phraseology all along, and a simple misunderstanding of what each church holds. This is not entirely satisfactory to many in Eastern Orthodoxy, and it is not considered in each church's competence to use a General Holy Synod to bring about communion. These Eastern Orthodox Christians hold that it would take another Great and Holy Council of every Eastern Orthodox Bishop together to reverse the Anathema, and this raises problems of its own.
The Catholic Church considers most forms of Protestantism to be heresy or at the least, in error (since they do not most do not believe in Apostolic Succession and thus their "rite" and ordinations are invalid); some Protestants are mutually hostile and consider Catholics, and sometimes the Orthodox, to be heretics (the exception to this rule are the episcopal Protestants such as Anglicans – particularly High Church Anglicans and Anglo-Catholics – and some episcopal Lutherans). The Catholic Church, since the Second Vatican Council, has been working harder to effect rapprochement among diverse forms of Christianity, these efforts have been met with wide-ranging responses.
Some religious groups are considered by all of the aforementioned to be unorthodox (or even arbitrarily cults, as they are less commonly called in Protestant circles), including members of the Church of Jesus Christ of Latter-day Saints or Mormons, Jehovah's Witnesses, Unitarians, and some of the more radical forms of liberal theology.
Inside each of these ecclesiastical communities there are issues that correspond to estrangement or refinements of perceived orthodoxy. For example, the Roman See often issues recommendations as to what practices it considers orthodox so as to curb excesses or deficiencies by its prelates. Some evangelicals are pursuing innovations that other, more conservative evangelicals consider unorthodox and term "neo-evangelical," "neo-pentecostal," or "fringe Charismatic."
In certain intellectual contexts, the terms "orthodox" and "orthodoxy" are used in an unfavorable sense, similar to that associated with "dogma" and "dogmatic". The implication is that orthodox beliefs are not rationally justified but are imposed by some overseeing body, such as the dominant group in an academic discipline. For example, the term orthodox economics is commonly used by critics to refer to the dominant approach to economics, which its supporters would more commonly call mainstream economics. In this sense, orthodox economics is commonly counterposed to radical or heterodox economics.