At least when used theologically, the term "mystery" does not mean something that is unknown and utterly unknowable. Rather, it means something we know about, if not completely, because God has revealed it to us. Hence, a mystery is not something that we are unable understand or discuss, "but rather that there is a transcendent aspect to [them] that humans cannot fully grasp" (Christ Jesus and the Jewish People Today: new Explorations of Theological Interrelationships xxiii).
To gain at least a partial grasp of the transcendent aspect of the mystery we celebrate today we need look no further than St Luke's Gospel:
"Hail, favored one! The Lord is with you." But she was greatly troubled at what was said and pondered what sort of greeting this might be. Then the angel said to her, "Do not be afraid, Mary, for you have found favor with God. Behold, you will conceive in your womb and bear a son, and you shall name him Jesus. He will be great and will be called Son of the Most High, and the Lord God will give him the throne of David his father, and he will rule over the house of Jacob forever, and of his kingdom there will be no end." But Mary said to the angel, "How can this be, since I have no relations with a man?" And the angel said to her in reply, "The holy Spirit will come upon you, and the power of the Most High will overshadow you. Therefore the child to be born will be called holy, the Son of God" (Luke 1:28-35)Even though I used this in my reflection for this Solemnity last year, I do not hesitate to invoke it again: "the Incarnational Event of God becoming human… is so earth-shattering that it enacts something akin to the psychoanalytic concept of trauma” on the world (Paul’s New Moment: Continental Philosophy and the Future of Christian Theology, 7).
Our second reading from the Letter to the Hebrews not only gives us a deep insight into the reason for the Word becoming flesh, it points us to the trauma of the Cross, that traumatic(/dramatic) event through which God, who is Father, Son, and Spirit, reconciled the world to Himself "once for all" (Heb 10:10), thus showing us Who God is for us contra mundum, but only in order to ultimately be pro mundum.
Another aspect, which emerges from our Hebrews reading, one that should never be lost on us, is how necessary is Jesus' Jewishness. After all, Jesus is not only consubstantial with the Father, but He is also consubstantial with His Blessed Mother.
In his essay "The Son of God Became Human as a Jew," published in the book I previously cited (i.e., Christ Jesus and the Jewish People Today), Hans Hermann Henrix pointed out,
to be meaningful, the Incarnation had to be rooted in centuries of preparation. Christ would otherwise have been like meteor that falls by chance to the earth and is devoid of any connection with human history (117)There is but one covenant between God and humanity, which, through Christ, is extended to all in fulfillment of the promise God made to Abraham in the immediate aftermath of the incident on Mount Moriah, where Abraham was prepared to sacrifice his only son, Issac, at God's command: "I will bless you and make your descendants as countless as the stars of the sky and the sands of the seashore; your descendants will take possession of the gates of their enemies, and in your descendants all the nations of the earth will find blessing, because you obeyed my command" (Gen 22:17-18).
So today, like the Archangel Gabriel in the presence of the young Miriam, who was, for reasons known to God alone, "filled with grace" "from the first moment of her conception," we bow before the great mystery of the Word made flesh for us.