A surprise "Dear Polly" message lands in my inbox from the Conservative Christian Fellowship wishing me a peaceful and blessed Easter and bringing with it a YouTube link to the prime minister's Easter message. David Cameron this week has been love-bombing the Church of England with a radio interview about his children's faith, a speech at an Easter reception for Christian leaders in Downing Street and an article in the Church Times.

It's mostly toe-curling stuff. Alastair Campbell never gave better advice than in warning politicians off doing God: it's horrible to behold. Sincere or not, they become as sounding brass and tinkling cymbals, as did Cameron talking of "our saviour", claiming "My government has a sense of evangelism" and even that "Jesus invented the big society 2,000 years ago". John Major's "back to basics" should flash warning lights. No politician can ever be Christian enough – an elastic creed stretching from the Tea Party to Christ's anti-capitalist hippy injunction to consider the lilies of the field and never think of tomorrow.

If Cameron's faith is, as he told the Guardian in 2008, "like reception for Magic FM in the Chilterns: it sort of comes and goes", it certainly came on strong this week. It matters little what he actually believes: let's not make windows into our leaders' souls, just watch what they do. Socially wet, economically dryer than bones is his style – yet his social liberalism winks on and off according to political exigency. For the Catholic Herald just before the election he was for cutting abortion times, anti assisted dying and he voted against lesbian motherhood without a father.

So why God now? His core message, "This is a Christian country", dog-whistles to key voters. Ostensibly, it soothes the noisy but electorally few affronted folk in the pews angry about gay marriage, whose fury he had underestimated. For them Cameron ladled out syrupy retro-visions of the C of E of his Oxfordshire upbringing, its liturgy and heritage, his love of early morning eucharist at his children's school's church. But his "Christian country" message is really whistling to the errant flock fled to Ukip. They may never attend, but the C of E is a cultural identity marker for those sharing Nigel Farage's distaste for foreign tongues on his commuter train.

Naturally, Cameron is careful to say "this is not somehow doing down other faiths". But those who feel threatened on account of their non-Christian faith won't find Christian branding reassuring. This week, an article on this site described how the far right is using pork to persecute Jews and Muslims, as Marine Le Pen stops schools serving non-pork options in the French towns she now controls. More horrible still, members of the Flemish Vlaams Belang party reportedly stormed into a school and forced pork sausages into children's mouths.

Such abuse by the right in the name of "secularism" outrages British secularists and humanists who stand with Voltaire, ready to die for the right of anyone's beliefs, so long as they don't impose it on others. At a time of anti-Muslim attacks, when Islamist extremism is feared for its terrorist potential, Cameron's "Christian country" is soaked in white nationalist significance. He has great verbal agility in sounding eminently moderate and reasonable while planting darker ideas. Behind a harmless love for country churches is a whiff of Lynton Crosby's culture war politics.

Politically, using the church as a St George's flag of convenience may not fly. The C of E is a confusing creature. Even while it tussles internally between conservative and liberal wings on gay marriage or female bishops, polls of its members show it's no longer the Conservative party at prayer: more vote Lib Dem and Labour. Look at the 40 bishops' raspberry of an Easter message to Cameron, with their strong rebuke against the "national crisis" of hunger so much worsened by his welfare policies. They know because their churches house the food banks used by almost a million people.

Even the Rev Steve Chalke has taken up arms, though his Christian organisation runs many Gove-approved academies and contracted-out social services. He was incandescent at Cameron's refusal to let a girl at one of his schools stay in the UK just a few months longer to take her A-levels. "The Bible is clear: it is our God-given responsibility to take care of the widow, the fatherless and the refugee," he said. Which is why it's unwise for politicians to tangle with God.

How does "Christian" play politically in Britain? I suspect most people, religious or not, shudder at politicians pitching their tents on church turf. The WIN/Gallup International survey finds the UK among the least religious: only a third say religion plays a positive role. Asked in the 2011 census "What is your religion?", 59% said Christian – surprisingly few as most people saw it as a question of culture rather than belief. Asked by YouGov more specifically, "Are you religious?", only 29% said yes and 65% said no.

That's good news for us humanists (I am vice-president of the British Humanist Association). Religion imposed on the rest of us is profoundly resented by the great majority. Take schools, where a third are under religious control. They take many fewer free-school-meals pupils and pews near good C of E schools swell unnaturally with new parents. Selection makes them popular, yet even so a majority want them abolished.

Or this: Lord Falconer's bill on assisted dying comes to the Lords in June, but it's almost certain to be killed off again by the bishops and the religious lobby, despite overwhelming public support over many years. The right to die when we choose is the last great individual freedom yet to be won. Every year thousands of people approach death through a torture chamber when they would prefer to die in peace at their own right time. This cruelty is ordained not by a "Christian country" but by a religious grip on parliament unrepresentative of the people.

Like all humanity, the religious are both good and bad. The C of E is good on food banks, bad on sex and death. Faith makes people no more virtuous, but nor do rationalists claim any moral superiority. Pogroms, inquisitions, jihadist terror and religious massacres can be matched death for death with the secular horrors of Pol Pot, Hitler or Stalin. The danger is where absolute belief in universal truths, religious or secular, permits no doubt. Politicians do well to stay clear of the realm of revealed truth. Cameron will win back few voters by evangelising for Britain as a "Christian country", while antagonising many.

• This article was amended on 18 April 2014. The earlier version referred to "The international Gallup survey" where it should have said "The WIN/Gallup International survey".