Abstract This article demonstrates the centrality of ‘law’ to the famous dictum of Prosper of Aquitaine, lex orandi lex credendi. In its various appearances since the fifth century AD, the relationship in Prosper’s dictum between worship and belief has been, at best, assumed. I propose that the nature of that relationship is actually explicit in the dictum itself. That is, Prosper’s invocation of law—‘lex’. In particular, I demonstrate the importance of that term from a modern and an ancient perspective: the modern notion of speech-act theory; and the historical context of the Codex Theodosius, newly promulgated at the time of Prosper’s writing. From those perspectives, Prosper’s dictum is newly perceived as words which do things. It is both an example of heresiology and also seeks to establish the law. On the one hand, there appears to be a normativity newly prescribed for doctrine (credendi) following the lead of worship (orandi). On the other hand, I argue that Prosper’s prescription is for something more lively than ‘mere’ ritual normativity, namely established custom. This, as I will show, emerges from the contribution of the familiar Roman pair ius and lex.
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