In the Orthodox Christian Faith the Seven Ecumenical Councils of the Byzantine period are still of great importance as the foundation for theological debates about the contemporary times.
The first Council was held in Nicea in 325 AD, led by Emperor Constantine who had just moved the Capital of the Roman Empire to Byzantium, and renamed it Constantinople (though not yet officially inaugurated). Main areas of discussions were the Nature of Christ, the Trinity, and the organization of the Church.
The Seven Councils were:
Nicea 325 (Established Rome, Alexandria and Antioch as main centers.)
Constantinople 381 (Constantinople was designated a new center, put on second place.)
Ephesus in 431
Chalcedon in 451
Constantinople 553
Constantinople 680-81
Nicea 787 (Proclaimed Icons to be venerated and kept in Churches)
Of important influence or present in the Second Council were St. Athanasius and the Cappadocian Fathers with Gregory of Nyssa, Gregory of Nazianzus, and Basil the Great.
This “second period” of the Orthodox Church, which started with Nicea in 325, ended in 843 with the “Triumph of Orthodoxy” when Empress Theodora ended an ongoing conflict started in 815 by Leo V the Armenian, and permanently reinstated icons in the Churches.