Hume's ‘Natural History of Religion’ offers a naturalized account of the causes of religious thought, an investigation into its ‘origins’ rather than its ‘foundation in reason’. Hume thinks that if we consider only the causes of religious belief, we are provided with a reason to suspend the belief. I seek to explain why this is so, and what role the argument plays in Hume's wider campaign against the rational acceptability of religious belief. In particular, I argue that the work threatens a form of fideism which maintains that it is rationally permissible to maintain religious belief in the absence of evidence or of arguments in its favour. I also discuss the ‘argument from common consent’, and the relative superiority of Hume's account of the origins of religious belief.