The origin of this solemnity of Mary, Mother of God, which the Church celebrates today, goes back to the early Church and to the controversies about the nature of Our Lord Himself. Some then, as now, taking a partial reading of Scripture, insisted He was just Spirit and that He had only seemed to be human. This heresy, called docetism (after the Greek meaning ‘to seem’) was one picked up by the founder of Islam, which to this day cannot see the truths the early Church saw. At the Council of Constantinople in 381, it was established as dogma that all three Persons in the Trinity were divine, but that did not settle the question of how the divinity of Christ related to His humanity? Here the question of the role played by His mother became central to the discussion within the Church.
All acknowledged she was the author of His human nature. Contrary to the propaganda spread in parts of the West after the Reformation (and here both Luther and Calvin stand innocent, as both had a great devotion to Our Lady), early Christians took a great interest in her; in the parlance of the faithful she was termed ‘Theotokos’ – that is ‘God-bearer’ or, more commonly, Mother of God. The patriarch of Constantinople in 430 was a monk from Antioch called Nestorius. He found such imprecision not to his taste. He wished to call her ‘Chistokos’ – the Christ-bearer. To many of the faithful that was to imply that Jesus was not really human. If Mary bore the human nature only, where did the divinity come in? That sounded like Theodore of Mopsuestia and adoptionism and opened the road to docetism. To Nestorius, to call he the God bearer was to miss out the fact that Jesus was human and opened the way to Appolinarianism. To the patriarch of Alexandria, Cyril, this was an affront.
Christ’s two nature were united in the one person of Jesus, and Mary could not have given birth to only one of them; Nestorius had committed the impiety of separating the two natures of Christ. Once the Word became flesh, the two natures were united in the one person. To the Antiochene ears of Nestorius this sounded as though Cyril was saying Christ only had one nature, and that the divine had absorbed the human. I have written elsewhere here on this and do not want to go over the same ground.
Cyril saw that if the human and the divine were not two natures united in one person, then the alternative was some form of docetism or subordinationism, and that was why, at Ephesus in 431 he fought so hard to secure the acceptance of the term ‘Theotokos’. One of the interesting phenomena here was that it was popular piety, the acceptance by the faithful, that Mary was the mother of God, which helped secure Cyril’s victory. As so often, the faithful saw clearly what it took learned theologians time to articulate. What had not been assumed could not have been saved, and since our human nature had been saved, it must have been assumed, which meant that in some way it was united in the one person of Jesus the Christ. Unfortunately, the question of the form of that union was not adequately answered, and it would take another Council to settle that.