Some years ago a colleague was admitted to hospital and a nurse came over to her bedside to fill in a form with her personal details. "Religion?" "None." Later my colleague overheard a pair of nurses gossiping about her. "She doesn't look like a nun."

The absurd presumption that everyone has a religion, almost as a part of their identity, to be ticked off on a form the way one ticks the boxes for sex, eye colour and known allergies, is ubiquitous in our society and it has yet to be expunged from our census forms.

The census of 2001 seemed to show that over 70 per cent of British people were Christian. This figure has been triumphantly and repeatedly invoked by politicians, prelates and apologists for religion, in apparently persuasive justification for a strong Christian presence in our governance and resource allocation. The census showed that we are still a Christian country, so it is claimed to be appropriate that all schoolchildren in England and Wales are required by law to take part in a "daily act of worship of a broadly Christian character"; right that 26 bishops should have seats reserved for them in parliament, where they influence political decisions in very Christian ways - on discriminatory faith schools, on abortion and on assisted suicide, for instance. Not just the unelected bishops: members of the Commons with an eye to re-election must heed the Christian voice and curry favour with the powerful Christian demographic. Seventy per cent of the population wants Christian policies, and 70 per cent cannot be gainsaid.

Many of us suspected that the vaunted 70 per cent hid an embarrassment of non-Christian, non-religious vaguery. "Well, our family has always been Christian and I was christened; I'm not a Jew or a Hindu and certainly not a Muslim, I love singing carols, Jesus was obviously a good person, just look at that gorgeous sunset, and there are more things in heaven and earth, Horatio, so . . . yes, I'd better tick the Christian box."

Naturally people are free to call themselves whatever they like, and if you want to call yourself Christian even though you don't believe in God and have only the haziest idea of Christian teaching, that is none of my business. However, it very much is my business, and every other citizen's business, if the recorded demographic strength of Christianity in the country is falsely inflated by a very broad and loose definition of what it means to be Christian, and if that swollen figure is then hijacked and exploited by partisans of a much more narrowly defined Christianity.

If you ticked the Christian box because (like me) you are moved to tears by Schubert and the Milky Way, and therefore consider yourself a "spiritual" person, your "spirituality" should not be used to justify bishops in the Lords, or "All Things Bright and Beautiful" in schools. Ditto if you ticked the box because (again like me) you have a nostalgic affection for the Book of Common Prayer and King's College chapel.

It was for this reason, among others, that many of us campaigned to have the religion question omitted from the 2011 census. Unfortunately we failed. The Richard Dawkins Foundation for Reason and Science (UK) therefore fell back on plan B. This was to commission a large and comprehensive opinion poll, in the week immediately following last year's census, to find out exactly what people who ticked the Christian box believe, what lay behind their decision to accept the Christian label, and their attitudes to Christian-based legislation. Can politicians and others plausibly quote the percentage calling itself Christian as ammunition in arguments about religion in schools, homosexual rights, abortion and voting by the Lords Spiritual?

The survey was done by Ipsos MORI in accordance with its strict rules to ensure accuracy and impartiality, and on 14 February we published the results in the form of two press releases and a link to the underlying data (all of which is now on richarddawkins.net together with links to the extensive press coverage). The main conclusions are very much as we suspected. First, although the official census figures have not yet been published, our sample suggests that the percentage that describes itself as Christian has dropped from 72 to 54 (plus or minus 2 points).

That is a significant finding in its own right, but more telling is how small a proportion of even that 54 per cent believes in Christianity in any sense that could reasonably justify giving Christianity privileged influence in public life. In all that follows, it is important to remember (a retired bishop with whom I debated on television this past week got this wrong) that the percentages quoted are not percentages of the population at large, but percentages of the 54 per cent who self-identified as Christian. I will call them "Census Christians".

To pick out a handful of Ipsos MORI's findings, only a third of the Census Christians ticked the Christian box because of their religious beliefs. Not counting weddings, baptisms and funerals, half of them hadn't attended a church service at all in the previous year, 16 per cent hadn't attended in the past ten years, and a further 12 per cent had never done so. Only 44 per cent of Census Christians believe that Jesus is the Son of God. Only a third believe that He was physically resurrected.

Why then did they think of themselves as Christian? Ipsos MORI asked them: "Which of the following statements best describes what being a Christian means to you personally?" Favourite, with 40 per cent, was "I try to be a good person" (well, don't we all, but some of us good people are Muslims, some are Jews, some are Hindus and rather a lot are atheists). Second favourite, with 24 per cent, was "It's how I was brought up" (indeed - I, too, was brought up Christian and I was baptised and confirmed in the Church of England, so I guess that makes me a cultural Christian). Only 15 per cent of Census Christians selected "I have accepted Jesus as my Lord and Saviour" and 7 per cent chose "I believe in the teachings of Jesus" as the best description of what being a Christian meant to them personally.

“I try to be a good person" came top of the list of "what being a Christian means to you", but mark the sequel. When the Census Christians were asked explicitly, "When it comes to right and wrong, which of the following, if any, do you most look to for guidance?" only 10 per cent chose "Religious teachings and beliefs". Fifty-four per cent chose "My own inner moral sense" and a quarter chose "Parents, family or friends". Those would be my own top two and, I suspect, yours, too.

The bottom line is that anybody who advocates a strong place for religion in government cannot get away with claiming that ours is numerically a Christian country as a basis for giving religion privileged influence. This conclusion is further borne out by part two of our Ipsos MORI survey. Census Christians were asked explicitly about their attitudes to various social issues as well as their views on religion in public life. Seventy-four per cent of them said that religion should not have special influence on public policy. Only 12 per cent thought it should. Only 2 per cent disagreed with the statement that the law should apply to everyone equally regardless of their religious beliefs (so much for the Archbishop of Canterbury's opinion that sharia law in Britain is "unavoidable", and for attempts to exempt Christians from compliance with equalities legislation). More Census Christians oppose than support the idea of the UK having an official state religion, and the same applies to the presence of bishops in the House of Lords.

Less than a quarter of Census Christians think state schools should teach children a religious belief. Sixty-one per cent support equal rights for gay people and 59 per cent support assisted suicide for the terminally ill, given certain safeguards. And for those MPs worried about re-election and the need to appeal to the allegedly powerful Christian lobby, 78 per cent of Census Christians say that Christianity has no or not much influence on how they vote.

Now finally to my joke title. During one of the many broadcast discussions of our survey, I used a vignette to illustrate how poorly acquainted Census Chris­tians seem to be with their Bible. Ipsos MORI asked them to identify the first book of the New Testament from a four-way choice of Matthew, Genesis, Acts of the Apostles and Psalms, plus "Don't know" and "Prefer not to say". Only 35 per cent correctly chose Matthew; 39 per cent didn't even guess, and the rest chose various wrong answers.

This is a truly stunning result. It is as though 64 per cent of those who self-identify as devotees of English literature were unable to pick out the author of Hamlet from a four-way choice of Shakespeare, Tennyson, Chaucer and Homer. It's not that ignorance of the sequence of the arbitrarily assembled canon of biblical scrolls matters in itself. The point is that this is an indicator of how utterly out of touch with Christian culture modern British people are, even those who signed on as Christian in the census.

How would a Christian apologist deal with this devastating result? Dr Giles Fraser was in the radio discussion, and he dealt with it by going to extraordinary lengths to deflect attention in another direction altogether. He scored what he obviously thought was a "Gotcha!" point by asking me whether I knew the full title of Darwin's On the Origin of Species. He meant including the long, Victorian subtitle. I confidently said I knew it because, rather surprisingly, I do. But I then had one of those momentary lapses of memory that become increasingly common around my age. I stammered out an approximation, but was unable to recall the exact wording until I was cycling home and no longer under the pressure of speaking in a radio studio: "On the Origin of Species By Means of Natural Selection: or the Preservation of Favoured Races in the Struggle for Life."

Canon Fraser (whom, incidentally, I greatly admire for his principled stand on the St Paul's tent protest) cannot seriously have thought the two cases were remotely comparable. The Census Christians were not asked to recite anything from memory, merely to pick out Matthew from a choice of four. Even if they had been asked to recite it, "Matthew" is just one word, while the full title of Darwin's great work has 21. The comparison is so inappropriate that, far from being a real gotcha, Fraser's diversionary tactic can only be seen as a measure of desperation, designed to conceal the embarrassing ignorance of their holy book shown by 64 per cent of Census Christians. In any case Darwin's Origin, I hope I don't have to add, is nobody's holy book.

The argument was the first in an astonishing series of diversionary moves in the national press this past week, some of them amounting to outright smear tactics. Perhaps the most absurd (of many) was the Sunday Telegraph hack who trumpeted a story that my remote ancestors had owned slaves in Jamaica. Well, that settles it: Dawkins is an atheist and his great-great-great-great-great-great-grandfather owned slaves. Gotcha! Case closed.

I can quite understand why those whose aim is to protect at all costs the privileged status of Christianity in UK public life would want to deflect attention from the very significant findings of this important Ipsos MORI research. These are facts, not opinions, they aren't going to go away, and no amount of game-playing or smear tactics or irrelevant digression is going to change them.

In modern Britain, not even Christians put Christianity anywhere near the heart of their lives, and they don't want it put at the heart of public life either. David Cameron and Baroness Warsi, please take note.