The sign was that there was a baby laid in a manger, wrapped in "swaddling clothes."
I want to linger there a second, because the Greek word there rendered by Luke is "σπαργανόω," which comes from the word "σπαράσσω." It's rightly translated "swaddling clothes," but it means to wrap up in rags -- to wrap up in torn fabric as in to "swaddle" a baby.
So the sign the Angels point to is this baby placed in a feeding trough wrapped up in rags -- rags which might be for babies, or for the wounded. Maybe for the dead.
So that's the sign at Christmas -- the sign at the birth of Christ: there's a baby born not in a temple or a castle or some lofty estate, but born so low as to be born with the poorest of the poor, in a stable among animals. And his garments are not fine cloth or soft linens: they're rags that are only good enough for a baby's back-end business or to wrap the sick and dying in.
So what to think of this? Here are three things to think about as you get on with your Christmas:
1. In that sign, it is clear that God is with us.
That's the ultimate promise YHVH makes to Israel -- when the savior is born, he will be "Emmanuel - God with us." And the Angels point out that the sign to the Shepherds is that this child is born of no account at all -- above no one in the world. This wouldn't be so true if Jesus had been born in Solomon's courts -- because as the Prince of the nation, he would be above so many and unreachable by them.
2. In that sign, it is clear that God loves us.
Jesus gave up Heaven for a stable so that, as he said to the disciples, he must go to Jerusalem and suffer many things at the hands of the elders, the chief priests and the teachers of the law, and that he must be killed and on the third day be raised to life. For us.
That's actually how we know what love is: the love of God was made manifest among us, that God sent his only Son into the world, so that we might live through him.
3. In that sign, God clears up everything He has been saying for the past 2 or 3 millennia.
In the past God spoke to our ancestors through the prophets at many times and in various ways, but in these last days he has spoken to us by his Son -- the one who is the radiance of God’s glory and the exact representation of his being, sustaining all things by his powerful word...God makes everything He said come true in the birth of a child in a barn because there was no room at the Inn.
All the ideas of blessing: rolled up in swaddling clothes.
All the ideas about being chosen by God: laying in a manger.
All those judgments and warnings in the Old Testament: now in the hands of a mother who admitted she didn't understand these things, but submitted to them and considered them in her heart.
All the promises: in poverty, to the least of these, with the least of these.
All the power: not considering equality with God something to be used to his own advantage, but rather, made nothing by taking the very nature of a servant.
Here in the manger is the very clarification of all God meant -- because he is here in this world as it is created.