Doubt is a promising subject of inquiry for historians. Its initial definition in the Oxford English Dictionary reads ‘[t]he (subjective) state of uncertainty with regard to the truth or reality of anything; undecidedness of belief or opinion’, which might be advocated as a necessary mindset for any historically inclined investigator embarking on research. Although not always articulated, historians constantly face the ‘state of uncertainty’ of knowledge of the past and the continuous need, therefore, to test the evidence. The compilers of the OED then, perhaps unwittingly, underscore the particular relevance of ‘doubt’ as a subject for ecclesiastical historians by further defining it as ‘uncertainty as to the truth of Christianity or some other religious belief or doctrine’. The prominent placing of this second definition acknowledges the reality that doubts about religious ideas and individual doctrines, if not faith itself, have long been conspicuous in human language, and not just when speaking about Christianity. Nonetheless, the means and the consequences of communicating doubt depend on, and are intensely revealing of, changing historical circumstance.