born of a woman, born under the law,
to ransom those under the law,
so that we might receive adoption as sons.
As proof that you are sons,
God sent the Spirit of his Son into our hearts,
crying out, “Abba, Father!”
So you are no longer a slave but a son,
and if a son then also an heir, through God.
-- Gal 4:4-7
Mary is truly "Mother of God" since she is the mother of the eternal Son of God made man, who is God himself.
- CCC 509
Throughout the Old Covenant the mission of many holy women prepared for that of Mary. At the very beginning there was Eve; despite her disobedience, she receives the promise of a posterity that will be victorious over the evil one, as well as the promise that she will be the mother of all the living. By virtue of this promise, Sarah conceives a son in spite of her old age. Against all human expectation God chooses those who were considered powerless and weak to show forth his faithfulness to his promises: Hannah, the mother of Samuel; Deborah; Ruth; Judith and Esther; and many other women. Mary "stands out among the poor and humble of the Lord, who confidently hope for and receive salvation from him. After a long period of waiting the times are fulfilled in her, the exalted Daughter of Sion, and the new plan of salvation is established."
-- CCC 489