Orthodox Images | The Theotokos
On the other hand, Christians believe God the Son is begotten of God the Father "from all eternity" (see Trinity and Nicene Creed), but is born "in time" of Mary. Theotokos thus refers to the Incarnation, when the Second Person of the Holy Trinity took on human nature in addition to his pre-existing divine nature, this being made possible through the cooperation of Mary.
Since mainstream Christians understand Jesus Christ as both fully God and fully human, they call Mary Theotokos to affirm the fullness of God's incarnation. The Council of Ephesus decreed, in opposition to those who denied Mary the title Theotokos ("the one who gives birth to God") but called her Christotokos ("the one who gives birth to Christ"), that Mary is Theotokos because her son Jesus is one person who is both God and man, divine and human. As Cyril of Alexandria wrote, "I am amazed that there are some who are entirely in doubt as to whether the holy Virgin should be called Theotokos or not. For if our Lord Jesus Christ is God, how is the holy Virgin who gave [Him] birth, not [Theotokos]?" (Epistle 1, to the monks of Egypt; PG 77:13B). Thus the significance of Theotokos lies more in what it says about Jesus than any declaration about Mary.
Within the Orthodox doctrinal teaching on the economy of salvation, Mary's identity, role, and status as Theotokos is acknowledged to be indispensable, and it is for that reason formally defined as official dogma. (The only other Mariological teaching so defined is that of her perpetual virginity). Apart from within Roman Catholicism, other Marian beliefs (for example, her sinlessness, the circumstances surrounding her conception and birth, her Presentation in the Temple, and her death) are expressed in the Church's liturgy but are not formally defined, and belief in them is not a condition for baptism. The Roman Catholic Church has formally defined as dogma two Marian teachings: her sinlessness (the Immaculate Conception) by Pope Pius IX in 1854, and her bodily Assumption into Heaven after her death by Pope Pius XII in 1950.
(From WikiPedia; Images from pravoslavie.ru)
