My thanks to the kind reader who sent me the programme from this year's Christmas carol service at the Old Royal Naval College chapel in Greenwich. It was written by the Rev Jules Gomes, chaplain of the college, and of Trinity College of Music, and also of the University of Greenwich.

Here is the good chaplain's Christmas message: "More Christians have been martyred for their faith in the last century than in any other period of church history. Yesterday's Herod is today's Richard Dawkins and Polly Toynbee, seeking the total extermination of all forms of Christianity. The great irony is that the greatest opposition to Christ comes from so-called broad-minded people who seek to ban Christmas so that people of other faiths are not offended."

Yes, it is that time of year when secularists, atheists and humanists become the Grinches who stole Christmas. As an honorary associate of the National Secular Society and president of the British Humanist Association, here is my cue to offer you all a rattling good Christmas "Bah, humbug!". Except, of course, it's all utter nonsense. No one is out to ban Christmas or Christianity - not atheists nor other faiths. Yet every year the same urban myths are repeated about the banning of Christmas by some pantomime villain local authority suffering from "political correctness gone mad". King Rat Christmas wreckers are unearthed, and every year these turn out to be garbage stories, but they are stored in the attic for another airing next December.

I had at least five calls from broadcasters this year inviting me to say it would be a jolly good thing if Christmas were rebranded Winterval. That myth began years ago when Birmingham city council tried to spread the festive season across the long winter - though it never replaced Christmas, which came with official celebrations in the middle of it. But the Winterval myth lives on. This year it was joined by this: "God rest ye merry people all, Let nothing go to waste, So let us all this Decemberval, Recycle now with haste." Although written by a vicar for Warrington's Christmas recycling campaign, watch Decemberval enter anti-Christmas demonology.

Christmas opinion polls stir the same pot. Theos, the religious thinktank, found a quarter of adults and over a third of 18- to 24-year-olds couldn't say where Jesus was born. Over half didn't know John the Baptist was Jesus's cousin; over a quarter didn't know who told Mary she was pregnant; and 78% had no idea where Mary and Joseph fled to escape Herod. Even the faithful were ignorant: only 36% of regular churchgoers got all four answers right. I regard this as awful. The loss of classical mythology has made much poetry, art and literature incomprehensible to most people. The loss of Christian mythology would make most European history and painting impenetrable. Secularists do not welcome ignorance as a substitute for declining faith.

Pursuing their annual "atheists are stealing Christmas" riff, a Sunday Telegraph survey of 100 schools found only one in five had a traditional nativity play this year, which is odd considering over a third of primaries are Christian. The sad truth is that some did no play, but others did Scrooge, Arabian Nights, Hansel and Gretel, or the Snow Queen, all also cultural treasures.

British Christians yearn to be martyrs, but frankly atheists are a pretty toothless substitute for lions. In a daft parliamentary debate this month on something called Christianophobia, Mark Pritchard MP accused the politically correct of banning religion from Christmas cards and advent calendars: "Many shoppers find it increasingly difficult to purchase greetings cards that refer to Jesus." Alas, market forces are probably rather stronger than humanist plots: with only 7% of people in church of a Sunday these days, Santa and the Snowman trump the nativity.

Evangelicals started a new myth this year that postage stamps with the Madonna and child are only sold under the counter: you have to ask for them, for fear of offending Muslims and Jews. Stuff and nonsense, retorted the Post Office. But you can bet this one will run and run - along with last year's myth that 70% of offices banned Christmas decorations for multicultural reasons. Another year it was the Red Cross banning cribs.

All this would just be seasonal silliness if it were not cover for a more sinister drumbeat. The right has taken to flying the "Christian" flag in ways that suggest none too subtly that foreigners - Muslims - are stealing our culture and traditions. "They" are stopping "us" celebrating Christmas and teaching Christian stories to our children. When Terry Sanderson, president of the National Secular Society, appeared on GMTV this week, although as usual he denied any atheist plot against Christmas, the theme in about 3,000 emails afterwards was: "We are not Muslims, our culture must not be silenced to avoid offending them."

The BNP has been quick to cash in. In the Christianophobia debate in parliament, the reported case of a BNP Christmas card was raised, "which portrays the holy family on the cover and inside are the words 'Heritage, Tradition and Culture'". Pritchard warned television firms: "The fear of violence from a particular faith group should not be grounds for hand-selecting or targeting other faith groups who may choose to protest peacefully." Fear of Muslim violence is killing off peaceful Christianity, he implies. But blaming mythical secular political correctness is usually a cover for more sinister suggestions that "our way of life" is under threat from foreigners.

Hastening to defend themselves against the charge, Trevor Phillips, chair of the Equality and Human Rights Commission, assembled imams, rabbis, Sikh and Hindu leaders to protest that they had no objection to Christmas, asserting that they sent Christmas cards, they liked cribs, and "it's a great holiday for everyone". Leave Christmas alone was the message, addressed again to the hypothetical politically correct secularists.

But we are innocent. It is the Christians who are stirring this dangerous pot, inventing non-stories, yearning for martyrdom - and worse, fermenting an outraged sense among the mainly secular population that they had better call themselves Christian because, as the BNP says, British "Heritage, Tradition and Culture" (read Kultur) are under threat from Muslims. While pretending to attack us, covertly these Christians stir resentment against immigrants.

As more faith trouble brews, it becomes ever more important not to ban religions, but to keep religion out of all functions of the state. It needs to be taught in schools, acted out in nativity plays, too, if they want - but without dangerously segregating children by their faith in sectarian religious institutions. And at last we have at least one political party leader brave enough to admit, like most people, that he doesn't believe in God.

As for secularists and humanists at Christmas, Dawkins himself told a disappointed BBC interviewer that he loves singing carols. And so do I. Not just Away in a Manger or Oh Little Town nostalgic childhood tunes, but all the enjoyably rich and strange theology of "Lo! He abhors not the Virgin's womb ... Veiled in flesh the Godhead see, Hail the incarnate Deity", and other such quaint delights.

Is it hypocritical to sing songs with words whose literal truth you do not believe? Any such sad edict would leave most great love songs, hymns and arias unsung. If the royal family can trill, with solemn faces and gladsome minds, "What can I give him, poor as I am?" then anyone can.

polly.toynbee@theguardian.com