The anthropologist Kate Fox calls it the "default religion rule". In her 2004 study Watching the English, she recalls eavesdropping on a conversation in a GP's waiting room as a mother helped her 12 year old daughter fill out a registration form. "We're not any religion, are we?" asked the girl, bemused by one of the questions, to which the mother replied, "No, we're not. Just put C of E."

But is this "rule" as strong as it was? Today's results from the 2011 Census suggest that the number of those self-identifying as "Christian" in England and Wales – the religion question wasn't broken down by denomination – has declined substantially from ten years previously [2]. Then it was 72 per cent. The latest figure is 59 per cent. There has been an almost equivalent rise of 10 per cent in those ticking the "No religion" box, but it's still only a quarter of the population. Even if the 7 per cent who declined to answer the voluntary question are counted as non-religious (which is unlikely to be the case) we're still left with more than two-thirds who declare a faith.

There's something for everyone here. The British Humanist Association was quick of the mark this morning, with Andrew Copson hailing [3] "a really significant cultural shift". He argued that the Census figures, while inflating the true figure, provided further evidence that religious practice and identity were in decline, "and non-religious identities are on the rise". But the Church of England has hit back. Its spokesman, Rev Arun Arora, pointed to the 59 per cent figure as evidence that [4] "the death of Christian England has been greatly exaggerated." He compared the fairly low membership of the National Secular Society with that of the British Sausage Appreciation Society. Which is fair enough, I suppose. There are, after all, many millions of people in this country who appreciate sausages without feeling the need to join a society to say so.

In truth, the Census figure reveals little that wasn't already known. Its main importance is political. Those who argue against social or political change (for example, against the introduction of same-sex marriage) or in favour of the special privileges enjoyed by the Church of England will breathe a sigh of relief that they can still point to the Census as proof that this is still a Christian country. For this very reason the BHA ran a high-profile campaign last year urging non-believers to identify themselves as such. It's not clear what difference this made: though there has been a huge fall in the number of self-declared Jedi, that in itself is not enough to account for the rise in the "no religion" figure.

Over the decade between the two most recent censuses, regular churchgoing has continued to decline, but at a slower rate than suggested by the figure for religious affiliation [5]. Other research (such as the poll carried out earlier this year for the Richard Dawkins Foundation [6]) suggests that only a minority of Britain's self-declared Christians have any deep knowledge of Christian doctrine or the Bible. The biggest driver of the decline in nominal Christian affiliation may be generational: younger people tend to have less cultural attachment to Christian traditions, and be less likely to adhere to the "default religion rule." But the terms of the debate have also been transformed. In 2001, religion had a relatively low public profile, although the fact that the question was added to the Census suggests that, even then, things were beginning to change. In the past few years, it has become difficult to avoid. Religion has entered the cultural and political debate in ways that would have been unimaginable just a few years ago.

The rise of public chatter about religion may not have been reflected in rising levels of active churchgoing. But it may well have contributed to a sharpening of people's religious identities. On the one hand, the prominence and popularity of the "New Atheists" has helped to turn non-belief into a positive identity. It has also confronted people with the notion that belonging to a religion involves at least some level of practice and/or theological belief, something that they may not have considered before. People who considered that they "belonged to" Christianity merely because they had been christened or married in church are now choosing to declare themselves non-religious.

Christianity has also become much more political. Debates about education, about the status of marriage, about abortion and medical ethics have became heavily dominated by questions of religion, and the dominant voices have often been religious ones (or, for that matter, anti-religious ones). There have also been attempts in some quarters to link Christianity with white ethnic identity, or with opposition to Islam. Some previously notional Christians will have been alienated by a church that often seems to embody regressive attitudes, and this may be reflected in the latest figures. But an opposite effect may also exist, whereby people of conservative views who are not personally religious nevertheless feel a strong identification with the "traditional values" that Christianity now seems to embody, even as many actual churchgoers find themselves out of sympathy with campaigners who would speak in their name.

So it's safe to assume that fewer people today adhere to the "default religion rule". To declare one's religion is now to make a conscious choice. For some people, no religion is now their default setting. For others, Christianity has gone from being a cultural given and become a political statement. But the one conclusion that it would be difficult to draw from today's Census figures is that they say much about actual belief.