Darkness is a fearsome thing: concealing, obscuring, bearing with it an almost tangible sense of oppression. The promise of the savior in Isaiah is nothing less than the promise to banish the darkness: the people who walked in darkness shall see a great light.
What does the Psalmist tell us?
I wait for the Lord, my soul waits,
and in his word I hope;
my soul waits for the Lord
more than those who watch for dawn,
more than those who watch for dawn. (Ps 130)
With dawn comes the light, and the end of the darkness, and that is why we wait and watch.
These days of Advent lead us, step by step, into that light, because to come into all at once is to be blinded by its brilliance. We are always told that Easter is the most important day on the calendar, because it is the promise of resurrection. Easter is the cause of our hope.
But Christmas is the cause of our glory. The incarnation is such a remarkable thing that it’s impossible grasp all at once. It is strong medicine for the fall of man, doing nothing less than divinizing flesh and enfleshing divinity.
God makes himself known in the most visible and powerful way possible. The theophanies of the Old Testament were mere foreshadowings of the incarnation. In these instances, God worked through creatures to make himself and his mighty presence known to man.
To make Himself fully known, however, he had to be visible and tangible. He had to be human. Then He could teach us. He could be our model. He could show forth his strength, goodness, power, and mercy for all humanity to imitate. And, ultimately, He could pay our debt.
He could not have done this in some other form or even through some creature, for He had to be both fully human and fully divine: the divinity (uncreated, eternal, and unchanging) showing forth the ideal, and the humanity (created, mortal, and mutable) showing what simple flesh could achieve.
By taking on flesh in the womb of Mary, He glorifies us all. We may look to Easter for our Hope, and it is right and proper to do so. But we look to Christmas for our Glory, because it is when God came among us and made our flesh–flesh that was the cause of so much distress and difficulty since man was first set down in Paradise–the very channel of our salvation.