An Ipsos MORI survey (pdf) from last summer found that belief in a One True God is less common than many assume:
Razib Kahn reflects on this finding:
Even in Saudi Arabia 25 percent of the population would not sign on to a very exclusive reading of their religion. This is not surprising to me. Very exclusive adherence to the proposition that all non-believers are damned is often hard to adhere to in any marginally cosmopolitan circumstance. Obviously there are people who will agree that Gandhi is in hell (this is a litmus test used to smoke out heterodox deviation in some fundamentalist Protestant churches in the USA), or that their close friend is going to hell, but when push comes to shove most people flinch. There seems to be a wide range in responses to this question about religious exclusivism, and I think that’s probably due to differences in priming.