Religious beliefs and delusions tend to be puzzling to those who do not share them, often violating established biological and physical principles. Indeed, some scholars have suggested that there is no meaningful difference between religious belief and delusion. This suggestion, however, has serious pragmatic limitations, as it effectively pathologises billions of people. We propose that a core research goal should be to develop a comprehensive and nuanced theory of belief formation – and malformation – that encompasses religious beliefs and delusions, yet elucidates the differences between them. Such a theory will recognise the importance of culture in psychiatric diagnosis, without ruling out the possibility of group delusions.