The Ecumenical Council of Ephesus was convened in 431 A.D, during the reign of King Theodosius II (Theodosius the Younger). 200 Pontiffs participated in the Council, with the aim of rejecting the false teachings of Nestorius, Patriarch of Constantinople. According to his teachings there were two independent, divine and human, natures in Christ, contrasting each other. Nestorius preached that Christ was born as a simple man and only later Divinity was settled on His Person. Therefore, the Holy Virgin Mary was not “Godmother”, but the mother of a simple man.
The Ecumenical Council of Ephesus condemned the teaching of Nestorius and adopted the teaching of Archbishop St Cyril of Alexandria, which stated that the divine and human natures of Christ do not exist separately, but are united without confusion. One Lord, one Jesus, one face and one united divine and human nature.
Additionally, they agreed that St Mary is not the mother of a simple man, but she is Theotokos (birth giver of God), as she gave birth to the Son of God.
St Cyril of Alexandria’s statement “One is the nature of the Incarnate Word of God”, was thus adopted by the Ecumenical Council of Ephesus.
The Armenian Church did not participate in that Ecumenical Council, but adopted its resolutions and ecumenical authority together with the previous Ecumenical Councils.