On the morning of the third day there was thunder and lightning, with a thick cloudover the mountain, and a very loud trumpet blast. Everyone in the camp trembled. Then Moses led the people out of the camp to meet with God, and they stood at the foot of the mountain. Mount Sinai was covered with smoke, because the LORD descended on it in fire. The smoke billowed up from it like smoke from a furnace, the whole mountain trembled violently, and the sound of the trumpet grew louder and louder. Then Moses spoke and the voice of God answered him. (Exodus 19:16-19)
When the God of the heavens tore open the heavens and came down to meet with his chosen people, he came as "a consuming fire" (24:17). "Our God comes and will not be silent," attests the psalmist. "A fire devours before him, and around him a tempest rages" (Psalm 50:3). God coming to Earth is not a welcome sight for most people. Just ask King David!
In the same sermon from 1928 which I quoted from earlier, Dietrich Bonhoeffer says much the same--that the fearful event of God-among-us should lead Advent to be a time of self-examination.
Perhaps, after all, Advent is a time for self-examination before we open the door [to Christ]. When we stop to consider, the contrast between those early Christians and us is extraordinary. They trembled at the thought of God coming, of the day of the Lord, when Jesus, "Judge eternal, throned in splendor," would shatter the complacency of all the world. But we take the thought of God coming among us so calmly. It is all the more remarkable when we remember that we so often associate the signs of God in the world with human suffering, the cross on Golgotha. Perhaps we have thought so much of God as love eternal and we feel the warm pleasures of Christmas when he comes gently like a child. We have been shielded from the awful nature of Christmas and no longer feel afraid at the coming near of God Almighty. We have selected from the Christmas story only the pleasant bits, forgetting the awesome nature of an event in which the God of the universe, its Creator and Sustainer, draws near to this little planet, and now speaks to us. The coming of God is not only a message of joy, but also fearful news for anyone who has a conscience.
It's difficult for me to keep this in mind, when I've got 27 Christmases of warmth and cheer behind me. But perhaps only when we consider this, pondering it in wonder and awe, will the child who becomes the Prince of Peace really be good news.