There is reason for genuine puzzlement about Hume's aim in ‘The Natural History of Religion’. Some commentators take the work to be merely a causal investigation into the psychological processes and environmental conditions that are likely to give rise to the first religions, an investigation that has no significant or straightforward implications for the rationality or justification of religious belief. Others take the work to constitute an attack on the rationality and justification of religious belief in general. In contrast to these views, I argue that Hume aims to establish two important claims in ‘The Natural History of Religion’. First, almost all popular religions, including popular monotheism, are deeply superstitious. Second, superstitious monotheism is incompatible with the variety of theism supported by the argument from design. This incompatibility puts significant pressure on the rational acceptability of popular religions.