From the first reading for Mass today: "Everyone who believes that Jesus is the Christ is begotten by God." (1 John 5:1)
To me this is the good news of Christmas. The Son is begotten of the Father from all eternity, "begotten, not made" as we say in the Creed. By his Incarnation, we are given access to that same relationship to the Father. The Holy Spirit conceives the Word in the womb of Mary and thus bends the perfect love and mutuality of Father and Son into the world. This bending out of the relationship of the Son to the Father allows us to come in. (This, not incidentally, is why we call Mary the Mediatrix of grace.)
By the birth of Christ, the Life of the Blessed Trinity Himself is opened to us, that we might become new creatures begotten of God, enjoying the same relationship that Jesus has with the Father, as daughters and sons in the Son.