Judging by the crowds in York, lots of people still have things to do before Christmas. Adverts show us images of perfect celebrations, with fabulous clothes and the finest food.

We can all be tempted into this conspiracy of the ideal.

On Advent Sunday it might feel that we must see the approach of Christmas in a particular way. That there’s a template we must follow to ‘get it right’. It feels like we are being told, one way or another, that we have to conform to this illusion.

But today the Advent liturgy should lead us to pause if we think we’ve got our celebrations all wrapped up.

Because Advent is here to shake up what we think we know. Advent is here to stir us out of the sleep of certainty and to discover, or rediscover, that God outmanoeuvres our determination to see the world in a particular way.

Picture those early Christians in the city of Rome. New converts, new communities of faith, close in time to the events of the Gospel. Yet even for these fresh-followers of the Way, St Paul needs to remind them: ‘you know what time it is… it is the moment for you to wake out of sleep… the night is far gone, the day is near’. We don’t have time to sleep, because what God wants the world to become is already breaking through – like the first glimmer of light in the East – God is calling us.

Our Great East Window behind me tells the story of the themes bound up with Advent.

It’s not a season for the faint-hearted. All of us pass through life, to different degrees, on auto-pilot. We decide that we know how our local streets look; the way people behave; the conversations we expect. Our world only becomes distinct when things change suddenly or new people enter our circle of friends. Compare, for example, when you visit a place for the first time, you drink it in. We look up and down and all around. Strangeness can stimulate our interest, our awareness.

By contrast the familiar can enter our blood and become the everyday; the routine and the expected. Scientists say that how we see the world is generated from memory far more than it is from sight. If you’ve been watching the recent BBC adaptation of the book telling the story of the birth of the SAS, you can see how easy it is to distract and confuse people when they think what they’re seeing turns out to be very different. It’s why spies are so successful. More often than not, we see what we expect.

Today all our readings challenge that familiarity.

It’s no surprise that Advent has a lot do with shaking and stirring. The Church is reminding us that our God is a God of surprises. The biggest of which comes at the end of Advent with a birth and a family far from home.

So what’s the most important characteristic for a Christian in the season of Advent? I think the answer is captured neatly, and with complete conviction, by Dietrich Bonhoeffer:

“The celebration of Advent is possible only to those who are troubled in soul, who know themselves to be poor and imperfect, and who look forward to something greater to come”.

Advent is a season when we are called to wake up to a world that can and will be changed. In these days up to Christmas we don’t settle for a world as if it’s all that could be. In the face of injustice and suffering we believe that it can be different – can be redeemed – and we ask God to guide us, to play our part in helping this Kingdom come. The words in Isaiah describe this emerging transformation as an end to violence: ‘nation shall not lift up sword against nation, neither shall they learn war any more’. It is a time when the world is changing, and we must dispel our own illusions to follow God more faithfully.

If we’re asleep it’s hard to hear that call, and work for that Kingdom.

Perhaps it wouldn’t be stretching the idea too far to say that Advent is like waking up through a cold shower. An experience that alerts all our senses and shocks us out of complacency. As dramatic as a flood, and as sudden as the arrival of an unexpected guest.

Advent demands that we are alert and attentive – prepared for a God who ‘scatters the proud in the imagination of their hearts’. Because we need to see past the reality we think is there, and begin to see our own lives, and the world around us, as God calls it to be.

The poet Stewart Henderson captures this with insight and economy in a poem entitled ‘Don’t Miss Christmas’. Because there’s a real risk, every year, that we seem hell-bent on missing what it’s all about – as Isaiah asks elsewhere: why ‘spend money on what is not bread, and … labour on what does not satisfy?’ In his own way Henderson sets this as a challenge for us all:

Don’t miss Christmas –

the magic of it all

our brittle, gift wrapped anthem

sleeps in a cattle stall

as the poor and lost and starving

weakly start to sing

it seems only desperate subjects

recognise their King.

We are given Advent to use. But we can only use it if we are prepared to see ourselves, and the world around us, with fresh eyes. To ask to be shaken out of complacency and made alive to the world as God calls it to be. Perhaps, as both Bonhoeffer and Henderson suggest, in this work, our need is far more important than our ability; our questions much more hopeful than our answers.