The Christmas season is unique in the Christian calendar for its emphasis on a sometimes neglected demographic: the angels.
Luke's account of the birth of Christ includes no fewer than three angelic appearances. In each case, they deliver a message: to Zechariah, to Mary, and to the shepherd's in the field. Their presence culminates with the appearance of the "heavenly host" sounding their refrain, "Glory to God in the highest."
In many ways, the presence and ministry of the angels reveals a humility similar to that of Jesus and Mary. They are delivering their final message: "In these last days [God] has spoken to us by his Son." And it is a message that entails their own demotion within the ordering of creation. While Christ is for a little while "lower than the angels," as a result of his Incarnation, he is given the name which is above every name--a position which we share by virtue of our brotherhood with him. In the mystery of salvation, we too are raised above the angelic hosts.
But rather than scorn the now-lowly incarnate Son of God, the angelic hosts responds in humble adoration. "Glory to God in the highest. The Incarnation is no scandal to them, but the vindication of the love of heaven.
In Christmas, then, the angelic hosts reminds us of the deep unity of creation, which is a unity that depends upon the joyful acceptance of displacement to those who are more worthy. It is a unity of joyful giving and humble receiving, of eagerly telling and raptly listening.
But above all, it is a unity established and confirmed by the worship of Jesus Christ, the Lord of all Creation. "Glory to God in the highest" is the refrain of all those who dwell within heaven. The joyful unity of Creation is accomplished when we join with the heavenly host in our worship of the Christ, the Holy One of Israel.
Christmas is an invitation to join with all of heaven in the worship of the Incarnate Lord. It is an invocation, a summons to dwell within the House of God--a house populated with creatures of all kinds. Perhaps no Christian poet expressed it more majestically or elegantly than Charles Wesley, with whose words I close:
Joyful, all ye nations rise,
Join the triumphs of the skies.
With the angelic host proclaim:
"Christ is born in Bethlehem"
Hark! The herald angels sing
"Glory to the newborn King!"
Update: Reposted from a few years ago, with minor edits.