Eastern Orthodoxy is one of thirteen eligible religions featured in the Gods & Kings and Brave New World expansions for Civilization V. It is an offshoot of Christianity that is introduced along with Catholicism and Protestantism in Brave New World.
Unlike the religions in Civilization IV, this religion has no technological requirements to be unlocked and neither unique buildings nor a unique missionary unit. However, the Cathedral religious building may be said to be close in spirit to Eastern Orthodoxy, although there is no gameplay relation of any kind between the two. Like all other religions, Eastern Orthodoxy can be founded directly after using a Great Prophet's Found Religion ability and only one civilization can be the founder of this religion.
The Eastern Orthodox Church is currently the second-largest Christian denomination, with an estimated 300 million adherents. The goal of Orthodox Christians is to draw nearer to God throughout their lives through theosis, a spiritual pilgrimage to become more "Christ-like."
The church traces its roots to the Great Schism. During the 9th and 10th centuries AD the Christian church under the Patriarch of Constantinople made significant conversions among the peoples of eastern Europe, including Kievan Russia and the Balkans. Doctrinal issues such as the filioque split and the authority of the Pope over the Patriarch in matters religious, exacerbated by the political and economic rivalry of Rome and Constantinople, led to a schism. The final breach between these two branches of Christianity is acknowledged as the sacking of Constantinople by the Catholic crusaders in 1204 AD. Moreover, after the fall of Constantinople to the Ottomans in 1453, the Eastern Church became even more isolated from Rome under the relatively tolerant rule of the Turks. The Orthodox Church, which saw itself as the true heir to the teachings and practices of the early Christians, flourished under the Russian Tsars. Although somewhat diminished by Communist rule and Balkan secularism, Eastern Orthodoxy has experienced a resurgence in recent decades and remains a vital aspect of Christianity.