The Gospels speak of Christ as both divine and human, and church Tradition was faced with the task of formulating a dogma on the unity of the divinity and humanity in Christ. This dogma was developed in the course of the Christological debates of the fourth to seventh centuries.
In the second half of the fourth century Apollinarius of Laodicea spoke of the pre-eternal God-Logos Who took human flesh; in his opinion, Christ did not possess a human intellect or soul. In the person of Christ divinity merged with human flesh, which together comprised a single nature. According to the Apollinarian teaching, Christ could not be fully consubstantial with humans as He was without a human intellect and soul. He was a ‘heavenly man’ who had merely assumed a human shell, not a complete earthly human being.
Diodore of Tarsus and Theodore of Mopsuestia represented a different tendency in Christological thinking. They taught that within Christ there existed two separate, independent natures which related to each other in the following way: God the Logos abided in the man Jesus of Nazareth Whom He had chosen and anointed and with Whom He had ‘come into contact’ and ‘cohabited’. The union of humanity with the Divinity was not absolute but relative: the Logos abided in Christ as in a temple. The earthly life of Jesus, Theodore believed, was the life of a human being in contact with the Logos. God from eternity foresaw the highly virtuous life of Jesus and in view of this elected Him as His organ and as the temple of His divinity. At first, at the moment of birth, this contact was incomplete, but as Jesus grew in spiritual and moral perfection it became fully realized.
In the fifth century Theodore’s disciple, Nestorius, Patriarch of Constantinople, followed his teacher in separating Christ’s two natures, making a distinction between the Lord and the ‘form of a servant’, the temple and the ‘One Who lives in it’, the Almighty God and the ‘man who is worshiped’. Nestorius preferred to refer to the Holy Virgin as Christotokos (the Birth Giver of Christ, the Mother of Christ) and not Theotokos (the Birth Giver of God, the Mother of God), for, he said, Mary did not give birth to the Divinity. Popular disturbance regarding the term Theotokos (the people refused to renounce this attribution of the Virgin Mary which had been sanctified by Tradition), together with St. Cyril of Alexandria’s powerful attack on Nestorianism, led to the convocation in 431 of the Third Ecumenical Council in Ephesus, which formulated (though not definitively) the Church’s doctrine on the God-man.
In speaking about the Son of God, the Council of Ephesus mainly used the terminology of St. Cyril, who taught not the ‘contact’ but the ‘union’ of the two natures in Christ. At the Incarnation God had appropriated for Himself human nature, while remaining at the same time who He is: although perfect and complete God, He had become a human being in the fullest sense. In order to counteract Theodore and Nestorius, St. Cyril constantly asserted that Christ was a single Person, a single Hypostasis. Thus Mary gave birth to the same Person as God the Word. Following this reasoning, St. Cyril thought that to renounce the title Theotokos would mean to renounce the mystery of the Incarnation of God, for God the Word and Jesus the man are one and the same.