In contrast with contemporary heresiological discourse, the Codex Theodosianus, a Roman imperial law code promulgated in 438, makes no systematic gendered references to heretics or heresy. According to late Roman legislative rhetoric, heretics are demented, polluted and infected with pestilence, but they are not seductive temptresses, vulgar ‘women’ or weak-minded whores. This article explores the gap between the precisely marked terrain of Christian heresiologists and (Christian) legislators. The first part gives a brief overview of early Christian heresiology. The second explores late Roman legislation and the construction of the heretic as a ‘legal subject’ in the Codex Theodosianus. The third turns to the celebrated account crafted by Pope Leo I of anti-Manichaean trials at Rome in 443/4, arguing that they should be understood as part of a much broader developing regime of ecclesial power, rather than as concrete applications of existing imperial anti-heresy laws.