A CLOTHING shop in Oxford market sells T-shirts with religious messages. Favourites include "I have found Jesus!" with, in small letters underneath, "He was down the back of the sofa all the time"; and "God loves everyone", with, in small print, "but I'm his favourite!" Suitable for Advent, there is also: "Jesus is coming! Quick, look busy."

Advent is that time in the calendar that marks the start of the Christian year by looking forward to the end of history. This is the moment in time when time will be no more; when, lo, he will come with clouds descending, with angels and trumpets in accompaniment, to wrap up the history of earth and fold it into the eternal history of heaven.

Traditionally, therefore, Christians have used Advent to prepare. The four weeks before Christmas are punctuated with acts of penitence, abstinence, and self-examination. But just when the whole world seems to be caught up in the consumerism of Christmas preparations, the Church has a hard task in trying to persuade us that this is a time to pause, and to think about the judgement that is to come.

Setting aside most of December as a period of penitence and holy preparation seems almost risibly counter-cultural. Most of us don't like churches that bang on about sin too much. In our therapeutically attuned culture, the very concept has been downgraded. Sin may induce guilt and shame. Such concepts, we are frequently assured, are paralysing and unhealthy.

A culture formed mainly out of desire and achievement may find itself in the grip of a subtle temptation - namely, to confuse sin with imperfection, with what we lack as people. To be sure, it is often helpful to be conscious of sins of omission and negligence. Yet a society that plays down the idea of serious personal and social sin, and even apparently unfashionable concepts such as original sin, does so at its peril. In ignoring the dark side of human nature, we risk collapsing into a falsely optimistic world-view that then struggles to cope with the reality of evil when it strikes. Rather than accept sin as commonplace, we have regarded the state as exceptional, and even as a private matter.

I suspect that part of the problem lies in language. Sin is a short, simple word. Its very accessibility has arguably played a part in the weakening of its power. Our older and arguably denser religious vocabulary preferred "trespass". The word captures the idea that lines have been crossed: that some of the things we say, do, and think are actually offensive, and grieve God.

Cranmer's majestic collect for purity understood that a great deal of sin is concealed inside us: "Almighty God, unto whom all hearts be open, all desires known . . ."

All desires known - and in the run-up to Christmas? Does God really know what I desire? Yes: to God, all hearts are indeed open, replete with their miscible emotions and motives. All of them are seen by the one who is returning. Yet the prayer goes on, in petition: "Cleanse the thoughts of our hearts . . ."

A prayer for the cleansing of desire seems an apt way to approach the clutter and gluttonous onslaught of the Christmas consumerfest. But it also captures something of the Advent hope: that light can pierce the darkness, purity trounce pollution, salvation overcome sin. Advent, then, is a serious time of preparation. For Christmas, but also for the rest of our lives, and beyond.

This is an edited extract from Thirty-Nine New Articles: An Anglican landscape of faith by Martyn Percy (Canterbury Press, £16.99 (CT Bookshop special offer £14.99  - Use code CT260  ); 978-1-84825-525-8).