Leo the Great is right to call the Nativity a mystery. The virgin birth, the shining star, the visiting magi, the announcing angels, the murderous ruler, and the supreme king converge in a moment that, truly understood, should bring us a joy that transcends gifts and feasts and winter ambiance and even that laudatory idol of familial affection.
The things which are connected with the mystery of today’s solemn feast are well known to you, dearly-beloved, and have frequently been heard: but as yonder visible light affords pleasure to eyes that are unimpaired, so to sound hearts does the Saviour’s nativity give eternal joy; and we must not keep silent about it, though we cannot treat of it as we ought.
For Leo's listeners, the Nativity was a familiar remembrance, one so familiar that they needed to be reminded how eternal were its joys. Now, I fear, we don't think about it at all.