The angel said to [Mary],
“The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the child to be born will be holy; he will be called Son of God.” (Luke 1:35)
In the Eastern Church, Mary the mother of Jesus is honored by the Greek word Theotokos, which means “God-bearer.” The word comes down to us in large part through the Council of Ephesus (held in the year 431), which had been struggling with the theology of Nestorius.
Nestorius was the devout, though doctrinally challenged, patriarch of Constantinople. He did not believe in the total union of the divine and human natures of Christ and insisted that Mary was the mother only of the human nature of Jesus. He is reported to have said, “I can never allow that a child of three months old was God.”
In response, the Council emphasized that there could be no division in Christ, and that Mary was the mother of Emmanuel (which means “God with us”). By using the term Theotokos, they were pointing out that the child which Mary had borne in her womb was indeed God. The following wonderful but mind-boggling thought comes from a letter Cyril of Alexandria wrote to Nestorius:
For although visible and a child in swaddling clothes, and even in the bosom of his Virgin Mother, he filled all creation as God, and was a fellow-ruler with him who begat him, for the Godhead is without quantity and dimension, and cannot have limits.
http://www.fordham.edu/halsall/basis/ephesus.html
What does all this ancient history have to do with us?
Besides the doctrinal issue, there is an important sense in which we are called to be God-bearers, too — though of course in a different way from that of Mary. Baptized into Christ, we dwell in him and carry the presence of God with us always, that same God who is “without quantity and dimension, and cannot have limits.” We are to be so totally turned toward God that our lives, radiating mercy and compassion, may bring forth Christ for a world desperately in need of hope.
As Meister Eckhart said, “The seed of God is in us… Pear seeds grow into pear trees, nut seeds into nut trees, and God-seed into God” — assuming, of course, that we cooperate with the Spirit of God as Mary did.
“Here am I, the servant of the Lord;
let it be with me according to your word.”
(Luke 1:38)