From the Holy Father’s Sunday Angelus reflection of December 18, 2011 (but most appropriate for today’s solemnity!). Emphases, in bold, are mine.
I would like to dwell briefly on the importance of the virginity of Mary, that is, the fact that she conceived Jesus while remaining a virgin.
In the background of the events at Nazareth is the prophecy of Isaiah. “Behold a virgin shall conceive and bear a son and call him Emmanuel” (Isaiah 7:14). This age-old promise has found superabundant fulfillment in the Incarnation of the Son of God. In fact, not only did the Virgin Mary conceive, but she did so through the Holy Spirit, which is God himself. The human being that begins to live in her womb takes the flesh from Mary, but his existence is derived entirely from God. …
…The fact that Mary conceived while remaining a virgin is, therefore, essential for the understanding of Jesus and our faith, because it witnesses that it was God’s initiative and above all it reveals Who is conceived. As the Gospel says: “Therefore the child to be born will be called holy, the Son of God.” (Luke 1:35). In this sense, the virginity of Mary and the divinity of Jesus reciprocally guarantee one another.
This is why that one question that Mary, “very upset,” addresses to the Angel, “How can this be, since I have no relations with a man?” (Luke 1:34) is so important. In her simplicity, Mary is wise: She does not doubt the power of God, but wants to better understand his will, to fully comply with this will. Mary is infinitely surpassed by the mystery, yet perfectly occupies the place that, at the very heart of it, she was assigned.
Her heart and mind are fully humble, and, because of her singular humility, God expects the “yes” of this young girl to achieve His purpose. He respects her dignity and freedom. Mary’s “yes” means both motherhood and virginity, and her wish that her everything be for the glory of God, and that the Son who will be born to her may be a gift of grace for all.
Dear friends, the virginity of Mary is unique and unrepeatable, but its spiritual significance concerns every Christian. It, in essence, is tied to faith: in fact, those who trust deeply in God, welcome Jesus and his divine life within through the action of the Holy Spirit.
—Pope Benedict XVI, December 18, 2011
(Image: painting by John Collier)