J.M.J. "Mary's most fundamental privilege is that of being the Mother of God. We do not mean she produced the divine nature, of course. But her Son is God, so she is the Mother of God. Similarly, Mrs. Jones shares only in the production of the body of her son John, not at all in the making of his soul. Yet we do not say she is mother of the body of John Jones, but of John Jones, the person. Pius XI quoted St. Thomas Aquinas with approval in saying that 'From the fact that she is the Mother of God, she has a sort of infinite dignity from the infinite good that God is.' (Lux veritatis, Dec. 25, 1931, citing St. Thomas, Summa Theologiae I. 25. 6. ad 4).
"Mary conceived her son by the power of the Holy Spirit (Luke 1:35). The Archangel first told her that her Son was to be the Son of the Most High. However, any devout Jew could be called a son of God. But there was more: the angel told her He would reign over the house of Jacob forever: right then she would know He was to be the Messiah, for Jews then commonly believed the Messiah would reign forever. Finally, the angel said He would be conceived when the Holy Spirit would 'overshadow' her. That word, she would know, was the one use to describe the Divine Presence filling the ancient Tabernacle in the desert (Exodus 40:35). Her Son was to be called Son of God 'for this reason'. So that He was the Son of God in a unique sense. From this alone she likely knew of His divinity, especially when she would add the words of Isaiah 9:5-6 that the Messiah would be 'God the Mighty'. Even though the Jews found that text hard, she, full of grace, would readily grasp it."
--The Reverend William G. Most, Ph.D. (1914-1999), "The Blessed Virgin Mary: Her Privileges and Relation to Christ and His Church"