Christian world in the context of "Byzantine commonwealth"

Cyril of Alexandria (Ancient Greek: Κύριλλος Ἀλεξανδρείας; Coptic: Ⲡⲁⲡⲁ Ⲕⲩⲣⲓⲗⲗⲟⲩ ⲁ̅ or Coptic: Ⲡⲓⲁⲅⲓⲟⲥ Ⲁⲃⲃⲁ Ⲕⲩⲣⲓⲗⲗⲟⲥ ⲡⲓⲥⲧⲩⲗⲗⲟⲥ ⲛ̀ⲧⲉ ⲡⲓⲛⲁϩϯ or ⲡⲓ̀ⲁⲅⲓⲟⲥ Ⲕⲓⲣⲓⲗⲗⲟⲥ; c. 376–444) was the Patriarch of Alexandria from 412 to 444. He was enthroned when the city was at the height of its influence and power within the Roman Empire. Cyril wrote extensively and was a major player in the Christological controversies of the late 4th and 5th centuries. He was a central figure in the Council of Ephesus in 431, which led to the deposition of Nestorius as Patriarch of Constantinople.

Cyril is counted among the Church Fathers and also as a Doctor of the Church, and his reputation within the Christian world has resulted in his titles Pillar of Faith and Seal of all the Fathers. The Nestorian bishops at their synod at the Council of Ephesus declared him a heretic, labelling him as a "monster, born and educated for the destruction of the church".

Celtic Christianity is a form of Christianity that was common, or held to be common, across the Celtic-speaking world during the Early Middle Ages. The term Celtic Church is deprecated by many historians as it implies a unified and identifiable entity entirely separate from that of mainstream Western Christendom. For this reason, Brown (2003) notes a preference for the term Insular Christianity. As Patrick Wormald explained, "One of the common misconceptions is that there was a Roman Church to which the Celtic Church was nationally opposed."

Some writers have described a distinct "Celtic Church" uniting the Celtic peoples and distinguishing them from adherents of the Roman Church, while others classify Celtic Christianity as a set of distinctive practices occurring in those areas. Varying scholars reject the former notion, but note that there were certain traditions and practices present in both the Irish and British churches that were not seen in the wider Christian world.

During the 6th century, Roman Emperor Justinian I launched a military campaign in Constantinople to reclaim the western provinces from the Germans, starting with North Africa and proceeding to Italy. Though he was temporarily successful in recapturing much of the western Mediterranean he destroyed the urban centers and permanently ruined the economies in much of the West. Rome and other cities were abandoned. In the coming centuries the Western Church, as virtually the only surviving Roman institution in the West, became the only remaining link to Greek culture and civilization.

In the East, Roman imperial rule continued through the period historians now call the Byzantine Empire. Even in the West, where imperial political control gradually declined, distinctly Roman culture continued long afterwards; thus historians today prefer to speak of a "transformation of the Roman world" rather than a fall of Rome. The advent of the Early Middle Ages was a gradual and often localised process whereby, in the West, rural areas became power centres whilst urban areas declined. Although the greater number of Christians remained in the East, the developments in the West would set the stage for major developments in the Christian world during the later Middle Ages.

Pelagianism is a Christian theological position that holds that the fall did not taint human nature and that humans by divine grace have free will to achieve human perfection. Pelagius (c. 355 – c. 420 AD), an ascetic and philosopher from the British Isles, taught that God could not command believers to do the impossible, and therefore it must be possible to satisfy all divine commandments. He also taught that it was unjust to punish one person for the sins of another; therefore, infants are born blameless. Pelagius accepted no excuse for sinful behaviour and taught that all Christians, regardless of their station in life, should live unimpeachable, sinless lives.

To a large degree, "Pelagianism" was defined by its opponent Augustine, and exact definitions remain elusive. Although Pelagianism had considerable support in the contemporary Christian world, especially among the Roman elite and monks, it was attacked by Augustine and his supporters, who had opposing views on grace, predestination and free will. Augustine proved victorious in the Pelagian controversy; Pelagianism was decisively condemned at the 418 Council of Carthage and is regarded as heretical by the Roman Catholic Church and Eastern Orthodox Church. For centuries afterward, "Pelagianism" was used in various forms as an accusation of heresy for Christians who hold unorthodox beliefs. However, more recent scholarship in both theology and religious studies has questioned how strongly Pelagius's beliefs differed from those of his opponents and from modern mainstream Christians.