The Third Ecumenical Council was convened in 431 A.D. in the city of Ephesus (in Asia Minor), during the reign of Emperor Theodosius the Younger (408–450). The Council was called to urgently address and investigate the false teachings of Patriarch Nestorius of Constantinople (428–431).

Nestorius openly contradicted the established doctrine of the Ecumenical Church by claiming that Jesus Christ, the Son of God, was not one Person (Hypostasis), but rather two separate persons—one divine and one human.

He further denied the title of Theotokos ("God-bearer") for the Most Holy Virgin Mary, asserting instead that she should be called only the mother of the man Christ, not the Mother of God. This teaching directly opposed one of the central dogmas of the Christian Faith: the unified divine and human natures of Jesus Christ in one Person.

According to Nestorius's doctrine, Christ was born as a mere man, and only later was somehow joined to the Godhead due to His holy life. This blasphemous view threatened the very heart of Christian teaching: that the eternal Word of God was truly incarnate in the flesh of the All-Pure Virgin Mary, and through His suffering, death, and Resurrection, redeemed humanity from sin and death, opening the way to the Kingdom of Heaven for all who believe and live according to His commandments.

Long before the Council was convened, Saint Cyril, Archbishop of Alexandria, had made multiple efforts to correct Nestorius through letters and theological reasoning. Despite these attempts, Nestorius remained obstinate and continued spreading his errors.

Recognizing the danger this heresy posed, Saint Cyril appealed to Pope Celestine of Rome, who, along with other Orthodox bishops, also tried to persuade Nestorius to abandon his false teachings. When it became clear that Nestorius would not repent and his influence was spreading, the bishops sought the emperor's permission to convene an Ecumenical Council. The Council officially opened on June 7, 431, the Feast of the Holy Trinity.

Over 200 bishops attended. Though Nestorius came to Ephesus, he refused to attend the Council sessions, even after being summoned three times. The bishops, therefore, proceeded without him to examine and condemn his teachings.

The Council sessions ran from June 22 to August 31. Among the prominent participants were Saint Cyril of Alexandria, Juvenal of Jerusalem, and Memnon of Ephesus. Pope Celestine I was unable to attend due to illness but sent representatives (papal legates) in his stead.

The Third Ecumenical Council ultimately condemned the heresy of Nestorius and affirmed the Orthodox doctrine: that Jesus Christ is one Person with two natures—divine and human—and that the Virgin Mary is rightly called Theotokos, the Mother of God, and Ever-Virgin.

The Council issued eight canons to guide the Church and formally adopted Saint Cyril’s “Twelve Anathemas against Nestorius.”