In the fifth century a long and difficult controversy developed over the true understanding of the person and nature of Jesus Christ. The third ecumenical council in Ephesus in 431, following the teaching of Saint Cyril of Alexandria, was most concerned to defend the fact that the One who was born of the Virgin Mary was no one other than the divine Son of God in human flesh. It was necessary to defend this fact most explicitly because some in the Church, following Nestorius, the bishop of Constantinople, were teaching that the Virgin Mary should not be called Theotokos—a term already used in the Church’s theology—because it was claimed that the Virgin gave birth to the man Jesus whom the Son of God had become in the incarnation, and not to the Son Himself. In this view it was held that there is a division between the Son of God born in eternity from God the Father and the Son of Man born from the Virgin in Bethlehem; and that although there is certainly a real “connection” between them, Mary merely gave birth to the man. As such, it was held, Mary could be called Theotokos only by some sort of symbolic and overly-pious stretching of the word, but that it is rather dogmatically accurate to call her Christotokos (the one who gave birth to the Messiah) or Anthropotokos (the one who gave birth to the Man that the Son of God has become in the incarnation).
Saint Cyril of Alexandria and the fathers of the council in Ephesus rejected the Nestorian doctrine and claimed that the term Theotokos for the Virgin Mary is completely and totally accurate and must be retained if the Christian faith is to be properly confessed and the Christian life properly lived. The term must be defended because there can be no division of any sort between the eternal Son and Word of God, begotten of the Father before all ages, and Jesus Christ, the Son of Mary. Mary’s child is the eternal and divine Son of God. He—and no one else—was born of her as a child. He—and no one else—was incarnate in human flesh from her. He—and no one else—became man in the manger in Bethlehem. There can be no “connection” or “conjunction” between God’s Son and Mary’s Son because they are in fact one and the same person. God’s Son was born of Mary. God’s Son is divine; He is God. Therefore, Mary gave birth to God in the flesh, to God as a man. Therefore, Mary is truly Theotokos. The battle cry of St Cyril and the Council in Ephesus was just this: The Son of God and the Son of Man—one Son!
-Fr. Hopko