It may be of interest to atheists to know how many gods they don't believe in. Let's call the number N.
Problems with counting gods
There are a lot of issues in determining N.
- Where do we draw the line? Views on god can be very different, even from people nominally of the same religion.
- N will be considerably greater than the number of religions, because a lot of religions are polytheistic.
- In the 150,000 years of human history there would probably have been religions that were never documented and have left no trace.
- There is some evidence that Neanderthals and Cro-Magnons ritualistically buried their dead. Do we include an estimate of possible cavemen gods?
- Gods sometimes moved between religions. For example, do we count Zeus and Jupiter as one or two gods?
- Do you take into account gods that no human has ever believed in? (Or imagined?)
- What about gods that aren't "real" (Saradomin from Runescape, the Tooth God from Wayside School gets a little stranger)
Estimating N from population
It is estimated that there are 6,700,000,000 people currently living on the Earth and the total number of people who ever lived is 102,000,000,000 (102 billion or 102 thousand million depending on where you come from). It could be argued that everyone's idea of god is different, so this is N. Or, at least, this could be used as an upper bound for N, except that many people were (or are) polytheists. However, if we accept there would be (sometimes quite large) groupings of people with essentially the same religious beliefs, this would lower the estimate for N.
If these two effects roughly cancel each other out, then N = 102,000,000,000 may be a good starting estimate.
That's a lot of gods.
Estimating N from religions
Adherents.com claims to have figures for 4,200 religious groups currently existing on Earth.
Using the ratio of current population to the total number of people who have ever lived, we get an estimate of 63,000 religious groups throughout human history. (Only Homo sapiens' religions are being considered. It may well be that other hominids believed in god or gods, but it would be pure guesswork to estimate the number of gods they believed in.)
The modern dominant (that is, have the most adherents) religions are monotheistic, but they are few in number. Wikipedia lists 309 Hindu deities. The ancient Hittites claimed to have 1000 deities in their pantheon. So for a rough estimate of the average number of deities per religion, we'll take the average of these 3 figures, giving 440 deities per religion.
This gives an estimate of N = 28,000,000.
How many gods do theists not believe in?
For monotheists, the number of gods they don't believe in will be N-1, which, of course, will be very close to N. If the estimate above is correct, then (in some sense) atheists and monotheists only differ by 0.000036% in their beliefs.
In fact, working to 2 significant figures, even for the ancient Hittites this figure is the same.