The theological term “Theotokos” is one of the most fundamental of Christian piety. Writes St.John of Damascus:
“We hold that God was born of her. We do not imply that the divinity of the Word received from her the beginning of its being; but we do imply that God the Word Himself, Who was begotten of the Father timelessly before the ages and was with the Father and the Spirit without beginning and through eternity, took up His abode in these latter days, in the Virgin’s womb and was made flesh and born of her, without change, for the sake of our salvation. For the holy Virgin did not bear mere man but true God; not only God but God incarnate; a God Who did not bring down a body from heaven, nor simply passed through the Virgin as a channel, but received from her flesh of like essence to our own and subsisting in Himself; for if a body had come down from heaven and had not partaken of our nature, what would have been the use of His becoming man? For the purpose of God that the Word become man was that the very same nature, which had sinned and fallen and become corrupted, should triumph over the deceiving tyrant and so be freed from corruption. Hence, it is with justice and truth that we call holy Mary the “Mother of God”. For this name, “Theotokos”, embraces the whole mystery of the dispensation”.
Saint John continues, ” We never say that the Holy Virgin is the Mother of Christ (Christotokos), because this appellation came about in order to do away with the title Theotokos, (Mother of God), and to bring dishonour on the Mother of God, who alone is in truth worthy of honour above all creation”.
The term Theotokos was rejected by the heretic Nestorios (Syrian theologian and Patriarch of Constantinople from 428-431) who thus split the two natures of Christ and referred to His Mother as “Christotokos” (the one who bore Christ the man) not Theotokos the one who bore Christ the God. Friend and counsel of Nestorios in his early years was the presbyter Anastasios who, in one of his sermons, said, “No one should call Mary, Theotokos, because Mary was a human being, and it is impossible for God to be born from a human”. This shook both clergy and laity who were always taught to speak of Christ as God and never to split His divinity from His humanity. Nestorios, however, instead of rebuking his favoured Anastasios, approved his sermon and he himself rejected the term “Theotokos”. The true faith was defended against this heresy of Nestorios by St.Cyril of Alexandria who made the motto of his Christological battles the fact that the Virgin is Theotokos. And he became the heart and soul of the Third Ecumenical Synod of Ephesus (431), which restored the dogma of the Theotokos forever with these words:
“We confess, therefore, our Lord Jesus Christ, the only begotten Son of God, consubstantial with the Father in divinity and consubstantial with us in humanity, because in him the union of the two natures came about. Hence, we confess one Christ, one Son and one Lord. And in accordance with the true meaning of this uncaused union, we confess the holy Virgin as Theotokos because God the Word was incarnated and became man through her, and through this conception united with Himself the temple that came from her…”.
by Fr.Evagoras Constantinides
“The Story of the Virgin”, InterOrthodox Press, Berkeley, CA 2009