After many centuries, theology's relationship to law remains a fraught question. Does law, like flesh, stand against grace and spirit, in the classic 'Pauline' dichotomies? These polemical dialectics, however, have not always pertained. Prosper of Aquitaine's formulation 'ut legem credendi lex statuat supplicandi' unabashedly held the notion of law together with theological language (words about God and to God). My recent research has demonstrated for the first time what law is doing with theology in Prosper's aphorism, in particular the invocation of the Roman Legal concept of ius over and above lex. However, whilst this indisciplinary work yields fresh and important insights, there is still the feeling of leaving theology short. That is for two main reasons. First, if the boundaries between law and religion in Prosper 'almost' cease to be metaphorical, then we have not yet thought what theology's contribution to law might be. And second, the interdisciplinary study does not yet reveal anything new about theology itself. We are none the wiser about how theological language function as part of theology's deepest concern, which must include God's words to us. This paper argues for two alternative positions on the question of interdisciplinarity, using Prosper's famous aphorism as a test-case. On the hand, interdisciplinary studies are never theology, unless what has been brought to theology has been subsumed to the theological task. On the other hand, Prosper's aphorism is a perfect example of theology's refusal to be hermetically sealed into academic boxes. This is not to allow interdisciplinary work in by the back door. It is, rather, more radically, to suggest that theology is the most important goal to which other disciplines inexorably lead. For theology to become, once again, a 'transformative voice', means to recover the bold speech - parrhesia - which so characterised some of the original ressourcement in theology a hundred years ago.