First Ecumenical Council - First Council of Nicea, A.D. 325

This council was called by the Roman Emperor Constantine the Great. It was in response to the heresy of Arianism, which said that Jesus was not divine, but merely human. The Nicean Council declared that Jesus was both human and divine and it denounced  Arianism as heresy. The Council also defined the first part of what would later be called the Nicene Creed. 318 bishops attended this Council.

Second Ecumenical Council - First Council of Constantinople, A.D. 381

This council was called by Roman Emperor Theodosius I. It was in response to the heresy of Macedonianism, which said the Holy Spirit was merely one of God’s powers and not a person like God the Father and God the Son. The Council defined the doctrine of the Holy Trinity: that God is three persons – God the Father, God the Son, and God the Holy Ghost. This doctrine along with other articles was added to the Nicene Creed. 150 bishops attended.

Third Ecumenical Council - Council of Ephesus, A.D. 431

This council was called by Byzantine Emperor Theodosius II, grandson of Theodosius I. It was in response to the heresy of Nestorianism, which said Jesus was merely a man in whom the Word of God dwelled (as in a temple). Nestorianism also taught that Mary, Jesus’ mother, was merely the mother of Christ, not to be called Mother of God. The Council declared that Jesus Christ is completely God and completely man (although without sin) and that Mary is rightly called the Mother of God. Furthermore, the Council declared that the Nicene Creed, defined during the first two Councils, was complete and never to be changed. 200 bishops attended.

Fourth Ecumenical Council - Council of Chalcedon, A.D. 451

This council was called by Byzantine Emperor Marcian. It was in response to the Monophysitism, which said Jesus’ human nature was transformed by his divine nature, making him divine and not human. The Council declared, as it did in previous councils, that Jesus was both fully human (though without sin) and divine. 630 bishops attended.

Fifth Ecumenical Council - Second Council of Constantinople, A.D. 553

This council was called by Byzantine Emperor Justinian the Great. It was called due to the persistence of the Nestorian and Monophysite heresies. The Council confirmed, again, the dual nature of Jesus Christ as both God and man. 165 bishops attended.

Sixth Ecumenical Council - Third Council of Constantinople, A.D. 680-681

This council was called by Byzantine Emperor Constantine IV. Like the previous Council, it was called to deal with the persistence of the heresies about the human and divine nature of Jesus Christ. The Council declared that Jesus was fully man and fully divine and that the two natures exist with “no confusion, no change, no separation, no division”. 170 bishops attended.

Seventh Ecumenical Council - Second Council of Nicea, A.D. 787

Called by the Byzantine Empress Irene, this Council considered the question of icons: art which depicted Jesus, the Father, the Holy Ghost, Mary, and the saints. This included crucifixes. Many Christians, particularly in the East, venerated icons. Others considered this to be idolatry and sought to destroy icons. These opponents are the source of today's word “iconoclasts” (Greek for “image destroyer”). The Council declared that religious icons are not idols, but only representations. Therefore icons could be used to venerate Father, Son and Holy Ghost, Mary, and the saints, and had to be respected. However, icons were not to be worshipped for themselves. 367 bishops attended.

Eight Council - Fourth Council of Constantinople, A.D. 879-880

Restored St. Photius the Great to his see in Constantinople and anathematized any who altered the Nicene-Constantinopolitan Creed, abrogating the decrees of the Robber Council of 869-870. This council was at first accepted as ecumenical by the West but later repudiated in favour of the robber council in 869-870 which had deposed Photius.

Ninth Council - Fifth Council of Constantinople, A.D. 1341-1351

Affirmed hesychastic (a mystical tradition of prayer in the Orthodox Church. It is described in great detail in the Philokalia, a compilation of what various saints wrote about prayer and the spiritual life) theology according to St. Gregory Palamas and condemned the Westernized philosopher Barlaam of Calabria.

 
These are the statements of faith defined by the Orthodox Church. Together with the Gospels and the Apostles’ Council of Jerusalem, they form the essential, shared faith of all orthodox Christians.