What wonderful language St. Ambrose used in this hymn. He calls Christ "a giant in twofold substance one". What a glorious paradox is this giant tiny baby, this one that is two, this child of a virgin.
"The Holy Spirit, 'the Lord, the giver of Life', is sent to sanctify the womb of the Virgin Mary and divinely fecundate it, causing her to conceive the eternal Son of the Father in a humanity drawn from her own." (CCC ¶485) Her sanctified womb became the new temple, the new ark of the covenant that would carry the Word of God into the world. In this hymn, when we sing that "God in his temple dwells below", we sing of Mary's womb as His human tabernacle, temple, and ark. We sing, too, of Christ coming in the Eucharist to dwell in every tabernacle throughout the world. Truly, Jesus comes to us through Mary; without her 'yes', there would be no incarnation and, thus, no Eucharist.
When we receive Christ into our hands or onto our tongue at Mass, like Mary we are receiving our Lord in a vulnerable state. Each time, we must ask ourselves if we are properly disposed to be His dwelling place.
Tomorrow, we celebrate the birth of Christ "whose advent sets thy people free". May the beginning of this Christmas season bring you all freedom, blessing, and great joy.