First, the circumcision.
Leviticus 12:3 instructs the "people of Israel" to circumcise every male child on the eighth day after his birth. I found two sources particularly helpful in understanding why it is important that Jesus was subject to this requirement:
"In his nativity, God becomes human; in his circumcision, God becomes a Jew.... Jesus' circumcision wasn't simply a rote observance of an ancient and somewhat macabre ritual but was rather a necessary step towards establishing his 'street cred' as a Jew and a descendant of David, without which he could not claim the role of Messiah. Just the idea of the Ruler of the Universe being bound by his own laws provides plenty of material for reflection." (Anthony S. Layne)
"God chooses Israel centuries before he reveals himself in Christ and invites them to prepare themselves to be the means by which he would reveal himself to the world in an extraordinary way. Israel, throughout the long ages of its history, glimpses what God will do (that he will become a man) but when this revelation finally happens, the people of Israel are taken by surprise.... circumcision was the sign that Israel had been set apart from other peoples and nations for a particular purpose. In Christ, that purpose is revealed. Israel is the means by which God will reveal himself in the world, and the means by which God does this is by becoming an Israelite himself—being born into an Israelite family, learning its culture and customs, speaking its language, and living its unique way of life." (Father Steve Grunow)
Now, Mary, the Mother of God.
This is a title that pushes many people's buttons. How could this be?! (This question echoes Mary's own first response when the Angel Gabriel announced her pending pregnancy, to which the Angel replied, "nothing will be impossible with God," Luke 1:37). God created Mary; how could she then be said to have given birth to Him?
Of course she didn't conceive and give birth to the one who made her, but she is truly and wholly, no more and no less, the mother of His only begotten Son. "The person in whom Mary's act of conception terminates," writes Reginald Garrigou-Lagrange, O. P., "is the Word incarnate, a divine person. The divine maternity, therefore, is a relation, of Mary to Christ and of Christ to Mary."
In Luke 1:43, Elizabeth exclaims to Mary, "And how does this happen to me, that the mother of my Lord should come to me?" The mother of my Lord! My Lord? My God! And Mary, she acknowledged, is His mother.
It's right there, in the Bible.
"Mary treasured all these things in her heart" (Luke 2:19).
And so do I.