Look: America isn't a Christian country. It just isn't.

Britain is. We have established Churches (of England and Scotland). The Queen's the head of the CofE. We have bishops in the House of Lords, for goodness' sake. As a Baptist who believes in the separation of Church and State I see the red mist descending when I think about this, but it's true. Even the bottom tier of local government is the parish, which is a Church thing.

More people go to church in America, sure. But if you want to know what a Christian country looks like, it's just across the Atlantic.

So why does this interminably protracted Presidential election seem to depend so much on the candidates' religion?

Every one of them has declared his or her Christian faith. Donald Trump, a serially married philanderer, is wildly popular with evangelicals and has declared himself "one of them", though evidence of this before his presidential run is rather scarce. Mike Huckabee was a Baptist minister. Hillary Clinton plays up her Methodist roots. Barack Obama is a preacher as well as a president: his Amazing Grace at the memorial for the Charleston funeral was mesmerising.

So far, so far enough: if it's genuine faith and people want to know about it, why not? It wouldn't fly in this country, where religious people are seen as odd or dangerous, but America's different.

Here's why not. Politicised religion is toxic religion.

America doesn't admit it, but at the moment it has a state religion in defiance of its own constitution. A 2014 Pew Research Center poll reported by the Washington Post found that Americans are less likely to vote for an atheist than any other type of candidate, including ones who have never held office, have had extramarital affairs, are in their 70s or are gay. At the moment, the Post report says: "Being identified as an atheist in the United States today is still such a major political liability that a candidate holding this position probably could not gain a major party's nomination for president or even the Senate."

See what this means?

If you want to run for office in the US you have to say you're a Bible-believing Christian whether you are or not. I don't impugn the statements of faith of any of the current candidates, but I'm pretty sure that's not helpful.

If you're a top quality, smart, experienced, patriotic American who happens to be an atheist – or worse, a Muslim – you can give up any idea of serving your country at the highest level if you have any kind of integrity.

Ah, yes, that 'Muslim' thing.

Republican candidates have got themselves into a heap of trouble over this. Ben Carson told NBC's Meet the Press on Sunday, "I would not advocate that we put a Muslim in charge of this nation. I absolutely would not agree with that.'' Trump refused to correct a man who said at a campaign rally that Obama was a Muslim and asked "When can we get rid of them?" He later described some Muslims as "a severe problem".

That's another thing about toxic religion: it becomes a communal identity marker, not a description of someone's relationship with God.

In the case of many American conservatives, this has led to an extraordinary re-writing of their country's history to give it a Christian spin.

One of the main culprits in this exercise in wishful thinking is a man named David Barton, sometimes described as historian. He has generated enormous enthusiasm among conservatives with his contention – with which no serious historian concurs – that America was founded as a Christian nation and that the constitution is a Christian document. Barton's book The Jefferson Lies, in which Thomas Jefferson was depicted as a devout Christian who wanted the United States to be a Christian nation, was withdrawn by its publishers Thomas Nelson after it was found to be, well, wrong.

Such is the appetite for this sort of thing that Barton's reputation survived. Not only is he a frequent speaker and commentator still, but he's just been appointed to a leading fundraising Political Action Committee (PAC) for Ted Cruz. There's much more about him on the blog of Warren Throckmorton, who has devoted considerable space to exposing him.

But you see the problem. If America's a Christian nation, in its origin and its identity, everyone who isn't – because they're atheist, Muslim, Hindu or whatever – is to that extent marginalised, not really 'one of us'.

Actually, the founding fathers were considerably wiser than that. They had the debate more than 200 years ago and settled it, as Throckmorton shows. America is a secular state, full stop. Your religion or lack of it is not an issue, in any civil context.

So why does it keep coming back?

There are two reasons: fear and opportunism.

Muslims have done terrible things in Iraq, Syria, Afghanistan and elsewhere. That's not because they're Muslims, it's because that's the religion the people living there belong to. Yes, Islam is sometimes used to justify and intensify conflict and atrocities. But the implication of the narrative pushed by people like Carson is that there's an integral relationship between being a Muslim and behaving like a savage. This is so obviously false that it's astonishing people are falling for it. But they are: Carson's Islam remarks led to a massive boost to his campaign finances and have seen his popularity surge.

The truth is that Muslims have far, far more reason to be frightened of Americans than Americans have to be frightened of Muslims. Just look at the relative body counts.

In his Decline and Fall of the Roman Empire, Edward Gibbon famously wrote: "The various modes of worship, which prevailed in the Roman world, were all considered by the people, as equally true; by the philosopher, as equally false; and by the magistrate, as equally useful. And thus toleration produced not only mutual indulgence, but even religious concord."

His splendid cyncism about religion ends with a note of optimism. We see things differently today.

What's useful about religion is that it generates fear. And fear gets you elected.

Follow @RevMarkWoods on Twitter.