In the history of Christianity, the first seven ecumenical councils include the following: the First Council of Nicaea in 325, the First Council of Constantinople in 381, the Council of Ephesus in 431, the Council of Chalcedon in 451, the Second Council of Constantinople in 553, the Third Council of Constantinople from 680–681 and finally, the Second Council of Nicaea in 787.
These seven events represented an attempt by Church leaders to reach an orthodox consensus, restore peace[1] and develop a unified Christendom.[2] Among Eastern Christians the Eastern Orthodox, Oriental Orthodox, and Church of the East (Assyrian) churches and among Western Christians the Roman Catholic, Anglican, Utrecht and Polish National Old Catholic, and some Scandinavian Lutheran churches all trace the legitimacy of their clergy by apostolic succession back to this period and beyond, to the earlier period referred to as the Early Church.
This era begins with the First Council of Nicaea in AD 325, convened by the emperor Constantine I following his victory over Licinius and consolidation of his reign over the Roman Empire. Nicaea I enunciated the Nicene Creed that in its original form and as modified by the First Council of Constantinople of 381 was seen by all later councils as the touchstone of orthodoxy on the doctrine of the Trinity.