Yet the notion that the Christmas story was a surprise, that Christianity is a surprise doesn't quite ring true.
If you're the god-of-this-age or believe in a philosophers-god (god = people but very big) then it's not what you'd think 'god' would be. This is a distinction worth remembering. But Luke's point in his gospel, along with the other evangelists, seems to be exactly the opposite - that this Jesus came as he did is exactly what we should have expected, after the LORD had spent all history setting the stage and writing the grammar of the gospel, laid out in the Old Testament. This is no new idea. The Christ had long been expected, and this Jesus is that Christ.
So, "a bit unexpected"? No, methinks not. Part of what makes Christianity so different from pop-religion, this is the way of the Triune God, it always was. No surprise if you have the books of Moses open and your eyes open. God does what he gives us reason to expect he would do. He gives and serves and blesses and loves, which is unlike us but very like him.
He has always been doing this, and he wrote it in Moses and the prophets. He doesn't do things unexpectedly - he's not unreliable and unpredictable - he shares his plans with his prophets. This isn't 'putting God in a box' but about knowing God to be one you can cling on to and whose love wont fail you. The Triune God can be trusted, and when God promised that people would find the Christ in Bethlehem, the Saviour, it wasn't a shock but exactly what he'd promised he would do - and his promise rings onward and outward, even to save stupid, arrogant people like me, to bring me to himself forever.
The unexpected thing to do with that is to receive this love, and pour myself out for others, which is senseless if you have a different gospel, but the only reasonable thing when it comes to the Christian gospel.
Come, Thou long expected Jesus, Born to set Thy people free;
From our fears and sins release us, Let us find our rest in Thee.