Mary is referred to as Theotokos (Θεοτόκος="God-bearer"). The traditional Eastern Orthodox Troparion (hymn for the day) of the Annunciation is:
Today is the beginning of our salvation,
And the revelation of the eternal mystery!
The Son of God becomes the Son of the Virgin
As Gabriel announces the coming of Grace.
Together with him let us cry to the Theotokos:
"Rejoice, O Full of Grace, The Lord is with You!"
Today is the day that Mary answered the question that is asked of all of us: For whom do I live? In her "yes" to her vocation to be "mother of God" Mary is our forunner in the Christian faith.
Blessed James Alberione writes:
"Mary carries out her apostolate: to give Jesus to the Father, to human beings, to heavn. She gave Jesus on earth: in her God was magnified, becoming also man and the Savior of all."
"Mary always gives Jesus, like a branch that always bears him and offers him to humanity: suffering, glorious and eucharistic; the way, truth and life of humanity. She is the apostle of Jesus - an apostle not only of words, but of mind, will and heart."