Great strides have been made in the study of John Cassian over the last half-century, and yet persistent concerns are still expressed in regard to his theological competence. Invariably, the standard against which Cassian is found wanting is Augustinian orthodoxy. And there is nothing new in this: these concerns were first expressed by Cassian's contemporary, Prosper of Aquitaine, especially in his polemic Contra collatorem. Prosper is the harbinger of medieval Augustinianism and this makes his claims seem almost inevitable in retrospect – but as this analysis will demonstrate, his evaluation and denunciation of Cassian is systematically flawed. Close attention to Prosper's work reveals not only that his criticism of Cassian was ill-considered; it also shows that Prosper's own theological insight is less penetrating than one might have hoped to find in the ‘first Augustinian’. The consequences for the study of Cassian, I suggest, are that Prosper's threadbare accusations no longer need to be entertained. But there is further significance in this finding: it is Prosper's polemic against Cassian that provides the categories that have regularly been used to evaluate the early reception of Augustinianism. If, as is shown in this paper, Prosper was an opportunistic controversialist who can be seen to have distorted, embellished and in some cases even fabricated his opponent's arguments, then we should be extremely reserved in using his polemic as the point of departure, not just for studying Cassian, but for studying the development of Augustinianism as a whole.