Orthodoxy
A separate article treats Eastern Orthodox Christianity.
Orthodoxy, from the Greek ortho ('right', 'correct') and dox ('thought', 'teaching'), is typically used to refer to the correct observance of religion, as determined by some overseeing body. Orthodoxy is opposed to heresy and schism. People who deviate from orthodoxy by professing a doctrine considered to be false are called heretics, while those who deviate from orthodoxy by removing themselves from the perceived body of believers, i.e. from full communion, are called schismatics. Not infrequently these occur together. 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 is a violation of orthodoxy that takes the form of abandonment of the faith, be it for some form of atheism or for some other faith. 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.
Religion embraces conceptualization of the divine and practice of worship, and adherents of all faiths represent to others how they perceive these things. There is a degree of openness, and an extent to which these elements are non-negotiable, in all religions. Tribal religions may involve cannibalising non-believers, or may be very open to theological discussion; while monotheistic religions adapt themselves to diverse cultures in manifold ways while yet not relinquishing certain precepts. Issues of tolerance and syncretism are distinct; a religion may tolerate another, neither oppressing nor adapting to it; a religion may permit itself to be absorbed into another; a religion may be outwardly intolerant while yet absorbing some teachings from another religion. A religion may be more tolerant of others at a given point in time than at another. These forms of cultural interplay impinge upon the extent to which a religion may or may not appear to maintain a consistent stance concerning its theology and practice.
Various groups have laid claim to the word orthodox as part of their titles, usually in order to differentiate themselves from other, 'heretical' movements. Orthodox Judaism focuses on a strict adherence to what it sees as the correct interpretation of the Oral Torah, dating from the strict reforms instituted under King Josiah in 622/621 BCE. Within Christianity, the term occurs in the Eastern Orthodox, Western Orthodox, and Oriental Orthodox churches.
The Eastern Orthodox Churches hearken back to what they see as the original forms of worship; for example, the Nicene Creed is used in its form as revised at the First Council of Constantinople in 381, in contrast to the Roman Catholic church, which use the Nicene creed with the addition of the phrase 'and the Son' (see Filioque clause). This emphasis on the use of the original "creed" is shared today by all "eastern orthodox" churches.
The Catholic Church considers the Eastern Orthodox to be in schism and therefore not in full communion with the Holy See. Some Eastern Orthodox churches in turn consider Catholics to be heretics. Confusingly, the term "Western Orthodox" refers to both the few existing churches in full communion with the Holy See whose practices are largely Eastern Orthodox and to certain parishes within Eastern Orthodoxy whose practices resemble Episcopalianism.
The Catholic Church considers Protestantism to be heresy; some Protestants are mutually hostile, and consider Catholics, and sometimes Eastern Orthodox, to be heretics. In some cases the term apostasy is applied within mutual invectives. 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, including Mormons, Jehovah's Witnesses, Adventists, Nestorians and Copts, and adherents of Liberal theology in general.
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 conservative Evangelicals consider unorthodox and call Neopentecostal, neo-Evangelical or even fringe Charismatic.
In English, the term "Oriental Orthodoxy" is sometimes used to refer to non-Chalcedonian eastern Christians, i.e. the Nestorians and Monophysites, though given the big difference in Christology between the two, the term is often used only for the latter. (This distinction between 'eastern' and 'oriental' is impossible in those other languages that use the same word for both.)
The term orthodox is also frequently used by Christians to refer to what they consider "mainstream" Christianity, as opposed to what they consider to be cults. This usage is especially popular among certain Protestant groups.