In a recent debate with Alvin Plantinga, Daniel Dennett claimed that belief in God is also this absurd. I would argue that it actually goes the other way: atheism is, in a sense, a conspiracy theory. I'm not referring here to the ridiculous claim that Jesus never existed. Of course, that is a conspiracy theory, but I'm thinking of the more basic claim of atheism: that God does not exist, that there is no supernatural, that the natural world is all that exists.
I say atheism is a conspiracy theory in a sense because there are important senses in which it is not. Thinking that all the theistic arguments fail or that the problems of theism outweigh those of atheism does not make one a conspiracy theorist. God's existence is not blindingly obvious, so to compare those who disbelieve in Him to those who think there is a secret cabal of evil Jews running the world is, in many ways, inappropriate. So I don't mean to imply that atheism is on a par with conspiracy theories in general; only when looked at in a particular way.
The sense in which atheism is a conspiracy theory is with regards to religious experience. Throughout human history people have had experiences of "something" beyond the physical world. In fact, this is one of the most common experiences that human beings have. The atheist thesis would require us to believe that virtually all of these experiences are completely illusory. I find this about as plausible as claiming that our experiences of the physical world are illusory. Of course there are differences: everyone experiences the physical world while not everyone has religious experiences; the physical world imposes itself on us constantly, while religious experiences are usually temporary; etc. Nevertheless, the sense of the supernatural, of a "beyond," can impose itself upon us to a much greater degree than the physical world.
Some might object that atheists are not positing any actual conspirators, so to call it a conspiracy theory is misleading. However 1) atheists claim our experiences of the supernatural are simply by-products of how our brains evolved. Evolution is responsible for our having these experiences and thinking they're veracious when they're actually not. So evolution is functioning, at least metaphorically, as a conspirator, even though it lacks something that most other conspiracy theories lack -- mindful intent. 2) My focus is not on the cause of the conspiracy theory but on the effect. Atheists, by claiming that religious experiences are a widespread illusion, are making the same claim as other conspiracy theories: 9/11 wasn't what it seemed to be; the Moon landings weren't what they seemed to be, President Kennedy's assassination wasn't what it seemed to be, etc. Of course, many things aren't what they seem, but to simply dismiss the experiences of billions of people as illusory seems no more reasonable than to dismiss all the eyewitness reports that the Pentagon was struck by a large airplane and assert it was a guided missile instead.
Another possible objection is that religious experiences are radically divergent and contradictory, and this should make us skeptical of their veracity. I would argue that 1) the disagreements have been exaggerated. There are, of course, differing aspects of them and even contradictions, but there is also much more agreement than atheists are often willing to admit. 2) The fact that everyone tells the same story (that there is something beyond the physical world) is more significant than the disagreement of the details. It's therefore strange to claim that the answer must lie in precisely the opposite direction. When eyewitnesses give contradictory accounts of a car accident, we are not justified in believing that no car accident took place. 3) So at most the differences between these experiences would justify skepticism toward a particular account, but not to the phenomenon as a whole. 4) Again, this objection would apply equally to our experiences of the physical world. There are accounts of physical phenomena that neither I nor anyone I know has personally experienced. Such accounts can even seem to contradict the phenomena I have experienced. It would not be rational for me to conclude that all accounts of the physical world are therefore bogus, and all the experiences of it illusory.
Because I can, I'll end with a quote by C. S. Lewis.
If you are an atheist you do have to believe that the main point in all the religions of the whole world is simply one huge mistake. If you are a Christian you are free to think that all these religions, even the queerest ones, contain at least some hint of the truth. When I was an atheist I had to try to persuade myself that most of the human race have always been wrong about the question that mattered to them most; when I became a Christian I was able to take a more liberal view.
(cross-posted at Quodlibeta)