36 And behold thy cousin Elizabeth, she also hath conceived a son in her old age; and this is the sixth month with her that is called barren: 37 Because no word shall be impossible with God. 38 And Mary said: Behold the handmaid of the Lord; be it done to me according to thy word. And the angel departed from her.
Gregory of Nazianzus: How is Christ related to David, since Mary sprang from the blood of Aaron, the angel having declared Elisabeth to be her kinswoman? But this was brought about by the Divine counsel, to the end that the royal race might be united to the priestly stock; that Christ, Who is both King and Priest, might be descended from both according to the flesh. For it is written, that Aaron, the first High Priest according to the law, took from the tribe of Judah for his wife Elisabeth, the daughter of Aminadab [Exod 6:23; Luke 3:33]. And observe the most holy administration of the Spirit, in ordering that the wife of Zacharias should be called Elisabeth, so bringing us back to that Elisabeth whom Aaron married.
Ambrose: Behold now the humility, the devotion of the virgin. For it follows, But Mary said, Behold the handmaid of the Lord. She calls herself His handmaid, who is chosen to be His mother, so far was she from being exalted by the sudden promise. At the same time also by calling herself handmaid, she claimed to herself in no other way the prerogative of such great grace than that she might do what was commanded her. For about to bring forth One meek and lowly, she was bound herself to shew forth lowliness. As it follows, Be it to me according to thy word. You have her submission, you see her wish. Behold the handmaid of the Lord, signifies the readiness of duty. Be it to me according to thy word, the conception of the wish.
Scripture from the Douay-Rheims Bible. Commentary from St. Thomas Aquinas, Catena Aurea, Vol. III (London: Rivington, 1843).