Orthodoxy is a collective term for the Eastern Orthodox Church and Oriental Orthodoxy. These two branches of Christianity use the term orthdoxy to express their belief to have an unbroken connection to the faith, doctrine and practices of the ancient Christian church. Both the Eastern Orthodox Church and Oriental Orthodoxy use the original form of the Nicene Creed developed at the First Council of Constantinople in 381, referred to as the Niceno-Constantinopolitan Creed.
The Orthodox Church is the other main branch of Chalcedonian Christianity that until 1054 was formally united with the Catholic Church, although the political divisions can be traced back to the crowning of Charlemagne as Roman Emperor by Pope Leo III in 800, which did not sit well with the Emperor in Constantinople. The nominal head of the Orthodox Church is the Ecumenical Patriarch of Constantinople, but he does not interfere in the affairs of other Patriarchs. Defining features in-game:
Head - The Ecumenical Patriarch
Holy sites - Lebedos, Constantinople, Mount Athos, Jerusalem, Antioch
- There is always an Ecumenical Patriarch, even if Constantinople falls.
- Multiple Patriarchs - the five Pentarchs and Autocephalous Patriarchs under Kings.
- Patriarchs can grant divorces.
- The Ecumenical Patriarch can grant the Invasion Casus Belli.
- Patriarchs can excommunicate Orthodox characters.
Mending the Schism (requires Legacy of Rome DLC) - Orthodox rulers who control the Byzantine Empire can mend the Great Schism, the separation of the Catholic and Orthodox churches. Doing so requires control of the five Pentarchs (Constantinople, Rome, Alexandria, Jerusalem and Antioch), religious authority of at least 90% and 2000 piety. Mending the Great Schism makes Catholic Christianity a heresy of Orthodox Christianity, and upon mending the Schism many Catholic rulers will convert to Orthodoxy.
Unlike the near-absolute ecclesatarcal power of Catholicism, Eastern Orthodoxy supports a more secular-over-religious playstyle. In most starts before the Latin Empire start, the most powerful Orthodox state by far is the Byzantine Empire, but Georgia and Bulgaria are also formidable. In Russian lands after 1066, there are also several small Orthodox dukes and counts.
If you're the Byzantine Emperor, then you get the fabulous luxury of having the Ecumenical Patriarch as your vassal - and having your very own pet Patriarch is an amazing thing indeed. However, the Patriarch is far less powerful than the Pope, as he has only jurisdiction over Greece, Anatolia, and other areas not covered by other Patriarchs and Pentarchs.