Andrew Brown's recent blog on atheism and class attracted a huge number of responses, including one from BristolBoy asking, "Has anyone in the UK ever carried out a study of the demographics of belief, with particular reference to the UK?"

The answer is yes. Last year we at Theos, the public theology thinktank, commissioned a large survey (2,000+ respondents) looking into attitudes to evolution, a/theism and a whole host of related topics.

One of the questions, adapted from an earlier BBC/ICM survey, asked people not simply what they believed (about God) but whether they had changed their mind, and by cross-tabulating these results with standard demographic questions, we can get a reasonably detailed picture of the class composition of atheism and theism in the UK.

The results can be read here but in summary the study found that lifelong theists ("I have always believed in God") are disproportionately from lower socio-economic grades (DE: semi-skilled and unskilled manual workers or those unemployed or on state benefits), whereas lifelong atheists ("I have never believed in God") are disproportionately from upper social grades (AB: higher or intermediate managerial or administrative professionals).

No surprise there. The default position in the UK (and seemingly in humans themselves) has long been belief in God, so you would expect theism to be a mass movement and atheism a more select one.

What is interesting – and surprising – is that "converts" to theism ("I believe in God now but have not always done so") are disproportionately from upper and upper-middle social grades (ABC1: as above plus supervisory, clerical, junior managerial or administrative professionals), whereas "converts" to atheism ("I used to believe in God but I no longer do so") are disproportionately from lower social grades (DE).

The same data can also be analysed according to final level of education. As education correlates strongly with social grade, you would expect similar findings, which is more or less what you get.

Lifelong theists are disproportionately made up of those with no academic qualifications, whereas lifelong atheists are disproportionately made up of those with a Bachelor's degree (but not a master's or PhD). Moreover, lifelong atheists are disproportionately underrepresented in the category of those who have "no academic qualifications". Lifelong atheists, in other words, are better educated than lifelong theists.

However, it's important to note that the same story is going on in education as with socio-economic grade. "Converts" to theism are disproportionately made up of those with a master's degree or above, and those with "no academic qualifications" are disproportionately underrepesented in this group, whereas "converts" to atheism are disproportionately made up of those with "no academic qualifications", and with BAs (but not MAs or above).

In short, the data seem to be showing two things. First, atheism has historically been a minority movement of better educated and higher-social grade individuals whereas theism has more affinity with the lower and lower-middle class and the less well educated. Second, this is changing, with new theists coming from a higher social grade and being better educated than new atheists.

There are a number of ways of explaining this, none of them very satisfactory. The grand theory would be that atheism is finally becoming a mass phenomenon, in the way that theism always has been and, in so doing, is acquiring some of theism's demographic characteristics. It sounds persuasive but the fact that converts to atheism and converts to theism almost perfectly balance out (8.3% vs 7.7%) rather undermines the idea. The actual size of each constituency is staying largely constant. Only the composition is changing.

On a less grand scale, the data suggest that the effect of vocal atheism over the last decade has been to reach successfully into previously uncharted demographic territory (witness The God Delusion's sales figures) but at the cost of losing some of its intellectual credibility (the critical review of The God Delusion in the London Review of Books, for example).

If this is happening, we might expect to see atheism become increasingly "religious" in its composition if not in its size. In a sense, that was precisely what the atheist bus adverts earlier in the year were attempting. As the Canadian novelist Margaret Atwood commented "'I understand that in Britain recently, some people paid to put atheistic slogans on buses – someone paid! That's religion! Once you're paying money to put slogans on things, well it's either a product you're selling, a political party or religion."

If atheism does succeed in breaking out of its higher-class, intellectualist confines, it is likely to do so at the cost of becoming more like a religion than its adherents would like.