"Glory to God in the Lowest" declares GK Chesterton in his wonderful Christmas poem Gloria In Profundis. In this one phrase he cuts through the religious baggage and gets to the heart of the season. The phrase "Glory to God in the Highest" is not only sung by the angels during the nativity story. It's also the first line of the Gloria in Excelsis which is said or sung in Catholic, Anglican, Lutheran and Orthodox churches across the world every week.

It is a familiar part of the worship of billions of people. But just when we are expecting the word "highest", Chesterton trips us up. Instead he inserts "lowest." In an instant he has conjured up the story of Christmas in microcosm.

The Messiah was hoped-for, longed-for and prayed-for by the people of God. In the Messianic prophecies that we have become very used to hearing at this time of year, the ruler who would come and bring freedom and peace and justice is presented for the most part as a strong and mighty leader.

Isaiah 9 contains the familiar words, "The government will be on his shoulders. And he will be called Wonderful Counselor, Mighty God, Everlasting Father, Prince of Peace. Of the greatness of his government and peace there will be no end. He will reign on David's throne and over his kingdom." This expected Messiah was clearly going to have to be a warrior, a politician and of royal position.

But the words a couple of verses earlier gave a clue that this wouldn't be the only way to recognise 'Emmanuel.' "For to us a child is born, to us a son is given..." In this small foretaste, we see that the baby born in humble surroundings in Bethlehem was to be the one of whom the prophecy had spoken.

A vulnerable baby, reliant on his parents, who were young and probably confused and homesick. Things were about to get worse for them as well. They would have to flee to Egypt.

But this set the pattern for Jesus' life. He confounded expectations.

You want me to make some water out of wine? Well it's going to be the best wine this party has tasted yet!

You want me to obey the letter of the law rather than healing a man on the Sabbath, do you? Well I'm afraid I see what's really important here.

You were expecting a militant uprising that would throw off the shackles of Roman oppression were you? Well, sorry. I've got something that looks a lot more like suffering and a lot less like victory for you. But in the end, my victory is infinitely better than beating Roman occupation. I'm going to beat death itself...

Jesus was in the business of confounding expectations and his birth as a baby in Bethlehem started the trend in a marvellous way.

We call it The Incarnation. The idea that God became one of us. People have been struggling to come to terms with this astonishing claim ever since. But it is the part of our faith that changes everything – and sets us apart. "What if God was one of us?" asked a hit record 20 years ago. "He was one us, 2000 years ago" comes our reply.

The Christian claim is that Jesus is God 'con carne', in other words, with flesh. In other faiths, the deity remains to an extent distant and it's the requirement of us as humans to work our way towards that god. But in Christianity, we can't work our way towards God through our actions – in fact the opposite is true. He comes to us in the same way that He came to us that first Christmas. He gets into the mess and the minutiae of our lives, rather than standing aloof and expecting us to make our way to Him.

As Chesterton said, "As men dive for sunken gem, pursuing, we hunt and hound it. The fallen star has found it, in the cavern of Bethlehem."