Short answers, pulled from the story.
Christianity became the official religion of the Roman Empire on the 27th of February 380, when emperors Theodosius I, Gratian, and Valentinian II issued the Edict of Thessalonica. The edict recognized Nicene Christianity as the state religion and declared those who did not follow it to be heretics.
The Edict of Thessalonica was a decree issued in 380 by emperors Theodosius I, Gratian, and Valentinian II establishing Nicene Christianity as the Roman Empire's state religion. It required all subjects to follow the faith as professed by Pope Damasus I of Rome and Peter, Bishop of Alexandria, and branded non-adherents heretics who could not call their gathering places churches.
The Council of Chalcedon was convened by Emperor Marcian in 451 to resolve disputes over the nature of Christ. Rejection of its conclusions led the majority of Christians in Egypt and many in the Levant to break from the imperial church; these communities, preferring Miaphysite theology, eventually formed the communion known today as Oriental Orthodoxy, including the Egyptian, Syrian, Ethiopian, and Armenian churches.
Justinian I, who became emperor in 527, claimed the right and duty of regulating the minutest details of worship and discipline and of dictating theological opinions to be held in the Church. He recognized five patriarchates covering Rome, Constantinople, Alexandria, Antioch, and Jerusalem, launched military campaigns to recover western provinces, and introduced the term orthodox into his legal code, the Codex Justinianus.
The Rashidun conquests, beginning in 634, eventually brought Egypt, Palestine, Syria, and all of Persia under Muslim rule by the late 8th century. The new Muslim rulers offered religious tolerance to Christians of all sects, and many populations in Egypt, Palestine, and Syria accepted Islam within a few generations. The conquests also eliminated the patriarchates of Alexandria, Antioch, and Jerusalem from effective power, leaving Rome and Constantinople as the only functioning centers of imperial church authority.
The East-West Schism culminated in mutual excommunications between Rome and Constantinople in 1054. Underlying causes included Rome's claim to authority over all other sees, Constantinople's rival claim expressed in the Patriarch's title of ecumenical patriarch, and the political break finalized in 800 when Pope Leo III crowned Charlemagne as a rival Roman emperor. Attempts to restore communion at the Second Council of Lyon in 1274 and the Council of Florence in 1439 were rejected by the majority of Byzantine Christians.