301: The Kingdom of Armenia becomes the first state to adopt Christianity
as its official religion.
313: Emperor Constantine issues the Edict of Milan, legalizing Christianity in the Roman Empire.
325: Constantine calls the First Ecumenical Council at Nicea to condemn the Arian heresy, which had taught that the Son was inferior to the Father.
330: Constantine dedicates the city of Byzantium as the eastern capital of the Empire or "New Rome," renaming it Constantinople after himself.
381: The Second Ecumenical Council at Constantinople publishes the text of the Nicene Creed.
431: The Third Ecumenical Council at Ephesus condemns the Nestorian heresy and approves the veneration of the Virgin Mary as Theotokos (God-Bearer or Mother of God). The Nestorians go into exile in the Persian Empire and become the Assyrian Orthodox Church of the East.
451: The Fourth Ecumenical Council at Chalcedon condemns the Monophysite heresy and affirms that Christ had both a divine and a human nature. The Armenian, Coptic, Ethiopian, and Syrian Orthodox Churches would remain Monophysite; the Greek Orthodox church and the Western church adopted the Chalcedonian or Dyophysite position. This Council also gives Constantinople equal standing with "Old" Rome.
529: Byzantine Emperor Justinian closes the pagan Philosophical Academy in Athens.
532-537: Hagia Sophia, the new cathedral of Constantinople, is built by order of Justinian.
622: Muhammed flees Mecca, beginning the Muslim era.
638: Jerusalem is conquered by Muslims, who would eventually control the entire Middle East, northern Africa, and Spain. The loss of the ancient sees of Jerusalem, Antioch, and Alexandria would contribute to isolating Rome and Constantinople as the two most important ecclesiastical centers.
730: Byzantine Emperor Leo III bans the veneration of images and relics, inaugurating the first period of Iconoclasm.
787: The Seventh Ecumenical Council, Nicea II, restores the veneration of images.
800: Pope Leo III crowns Charlemagne Emperor of the West, the first one since Constantine.
815: A council called by Emperor Leo V again bans images, inaugurating the second period of Iconoclasm.
843: Byzantine Empress Theodora restores the veneration of images on the First Sunday of Lent, which became known as the Sunday of Orthodoxy.
858: Emperor Michael III deposes Patriarch Ignatius and installs Photius, a learned layman.
862: Byzantine Emperor Michael III ("the Drunkard") sends Constantine (later Cyril, died 869) and his brother Methodius (died 885) to Moravia to teach Christianity to the Slavs. They translated the Bible and other religious writings into Old Church Slavonic. These "Apostles to the Slavs" also had the support of the Popes of Rome, Hadrian II and John VIII.
867: Rivalry between Greek and Latin missionaries to Bulgaria prompts Photius to convene a council that deposes Pope Nicholas III, and attacks Western notions of papal primacy and many Western practices, including the insertion of the word "filioque" into the Nicene Creed. The Eastern church now dates the East-West split from this event (the Photian Schism).
869: Emperor Basil I "the Macedonian," who had murdered Michael III, seeks papal support by deposing Photius and restoring Ignatius. A council held in support of Ignatius is considered by the West only to be the Eighth Ecumenical Council (Constantinople IV).
877-80: Ignatius dies, Photius restored as Patriarch, legates of Pope John VIII accept him on condition that Bulgaria be returned to the Latin church, without really settling the theological issues.
988: Prince Vladimir is baptized and marries a Byzantine princess; Russia becomes a Christian nation.
1054: Western Cluniac monks take over Greek monasteries in southern Italy, Byzantine patriarch Michael Caerularius responds by closing Latin churches in Constantinople. Pope Leo IX sends Cardinal Humbert (considered an expert on Eastern Christianity) to Constantinople to work things out. Not knowing Leo IX had died, Humbert excommunicates Patriarch Michael in Leo's name, asserting that the Eastern church had removed "filioque" from the Creed. The Western church traditionally dates the East-West split from this event (the Great Schism).
1098: Beginning of the First Crusade, in which the Western Crusaders had an uneasy alliance with Byzantine Emperor Alexius.
1204: The armies of the Fourth Crusade, unable to conquer any Muslims, sack Constantinople to pay off their war debts to Venetian bankers, rampaging through the city for three days and setting up a Latin Empire.
1261: Emperor Michael Palaeologus reconquers Constantinople for the Byzantines.
1439: Byzantine Emperor John VIII, hoping for Western military aid against the Turks, travels to Italy and negotiates a reunion of the Eastern and Western Churches at the Council of Florence. When he returns East, leaders of the Greek Orthodox Church refuse to accept the reunion.
1448: Prince Basil II of Russia imprisons Bishop Isidore of Moscow, a Greek, for accepting the reunion of Florence. The Russian Orthodox Church declares its organizational independence from Constantinople and elects the first native-born Russian bishop, Jonas I.
1453: Turkish armies conquer Constantinople, bringing Byzantine Empire to end.
1589: Patriarch Jeremias II of Constantinople raises Metropolitan Job of Moscow to the rank of Patriarch of Moscow and of All Russia, making him the head of the largest Orthodox church. Moscow would come to be called "the Third Rome."
1596: At the Union of Brest-Litovsk, several million Ukrainian and Byelorussian Orthodox Christians, living under Polish rule, leave the Russian Orthodox Church and recognize the Pope of Rome, without giving up their Byzantine liturgy and customs. This was the beginning of what is variously known as the Uniate, Eastern Rite Catholic, or Greek Catholic Church.
1652-8: Patriarch Nikon of Moscow revises liturgical books to bring them into conformity with the Greek Orthodox liturgy. Opponents of this reform were excommunicated from the Russian Orthodox Church and become known as Old Believers, which are now divided into several sects. These excommunications were rescinded in 1971.
1917: Russian Revolution: Bolsheviks overthrow the Russian Czar, depose the Patriarch of Moscow and disestablish the Russian Orthodox Church.
1965: Pope Paul VI and Patriarch Athenagoras I of Constantinople mutually nullify the excommunications of 1054.
1990s: The end of Communism in Eastern Europe allows the Orthodox churches to re-emerge.
Orthodox vs. Catholic Nationalities