jats:pThis article examines the connections in late antique Christian thought between the ideas that heretics were inspired by the devil, and that the devil was a liar. It begins by showing that the association of the devil with lies was founded on scriptural exegesis, and that Scripture was regularly deployed in heresiologies to cement the links between the devil as ‘father of lies’, and heretics and schismatics as liars in Satan's image. It then offers a detailed case study of when, where and how accusations of direct and indirect diabolical dissimulation were made by the opposing parties of the ‘Donatist controversy’ in polemical texts produced primarily for their own side. The final part considers how these accusations were modulated in invented textual dialogues and in oral debates between the two sides, showing how direct accusations of diabolical activity made against opponents were often eschewed for more subtle insinuations of diabolical association.</jats:p>