Of all the blows dealt the secularisation thesis – the idea that nations get more secular as they get more modern – over recent decades, the demographic one is the most brutal.

Wherever you go in the world, the religious outbreed the non-religious. Worse, the seriously religious outbreed the mildly religious. The earth is set to be inherited by fundamentalists. So runs the argument of Eric Kaufmann, whose book, Shall the religious inherit the earth? is published this week.

Kaufman is no idle peddler of Eurabian panic, the idea that Europe is being overrun by hoards of Muslims who breed like rabbits and are all itching to impose sharia on us. A Reader in politics at BirkbeckCollege, with serious demographic credentials, Kaufmann has been working on population projections for years. His conclusions are that many of the religious-based population projections are wildly out, animated, no doubt, by fear, but that the data do back “at least part” of the Eurabia thesis. Europe will be far more Muslim in the 21st century than it was in the 20th.

More generally, theology emerges as one of the most accurate indicators of fertility, far better than religious, denominational or ethnic identities. Ultra-Orthodox Jews have a three-fold fertility advantage over liberal Jews in America and UK. The most devout Muslim women in Europe are 40% more likely than the least pious to bear three or more children. Fundamentalist and evangelical Christians consistently have more children than their liberal counterparts.

There are big questions here. Demography is not an exact science and what has happened in the past is not necessarily the best guide to what will happen in the future. Indeed, immigration, one of the key inputs into demographic equations, is notoriously difficult to predict.

And then there is the de-conversion rate. My dutifully having five “Christian” children (please note the inverted commas) will make precious little difference if puberty sees them discover the delights of sex, drugs and atheism. Kaufmann’s predictions take this into account, by incorporating the extent to which children adopt their parents’ faith, but this rate can change significantly over time.

Such caveats aside, it is seems highly unlikely that any one religious group, not even ‘fundamentalists’ will dominate British or European society in the next half-century, and that may be an even more ominous prospect than a world in which fundamentalists war against one another or against secularists. We are heading not for Eurabia but “Pluropa”.

America’s genius has been the way it has been able to welcome and assimilate people with different geographical, ethnic and religious attachments, allowing them to maintain those attachments whilst insisting, largely successfully, on loyalty to the host nation. Britain, like most other European countries, does not have such a track record. Living with difference – real and substantial, as opposed to cosmetic and superficial, difference – is not something we are used to. But if Kaufmann’s analysis is right, we need to learn how to, fast.

Nick Spencer is Director of Studies at Theos.