In a letter to the monk Patricius, Philoxenus told a cautionary tale about the downfall of the monk Adelphius. He was said to have accepted a Satanic vision of the Holy Spirit, abandoned ascetic labour and become the founder of the heresy of the ‘Messalians’. This article places Philoxenus’ account against the longer background of the invention of ‘Messaliainism’, and in particular of Adelphius as Messalian heresiarch. It shows how Philoxenus drew on traditions about monks receiving Satanic visions found in ascetic literature. It also demonstrates that Philoxenus’ story reflected polemical claims that the Messalians, like other heretics, were inspired by demons and Satan.