The Council of Ephesus, 431

The heresy of Nestorius which posited a double personality, a divine and a human, in Christ, found its greatest opponent in St. Cyril of Alexandria. It was the latter’s dogmatic letter to the heresiarch Nestorius that the Council of Ephesus, the third ecumenical council, adopted as an expression of orthodox Catholic belief when it met in its first session on July 22, 431. Since the God-man was one Divine Person, his mother could rightly be called the Mother of God. This appellation of the Virgin Mary had become a focal point of the dispute between Nestorius’s followers and the Catholics.

For we do not say that the nature of the Word became man by undergoing change; nor that it was transformed into a complete man consisting of soul and body. What we say, rather, is that by uniting to himself in his own person a body animated by a rational soul, the Word has become man in an inexpressible and incomprehensible way and has been called the Son of man; not merely according to will or complacency, but not by merely assuming a person either. And we say that the natures that are brought together into true unity are different; still, from both there is one Christ and Son; not as though the difference between the natures were taken away by their union, but rather both divinity and humanity produce the perfection of our one Lord, Christ and Son, by their inexpressible and mysterious joining into unity…

Jesuit Fathers of St. Mary’s College, “The Church Teaches”: Documents of the Church in English Translation