Gnosticism is a term derived from ancient Christian heresiography, generalised during the twentieth century by historians of religion to describe various ancient sects resembling one another in their doctrines and subsequently applied, without justification, to certain non-Sunnī Islamic groups, particularly the Ismāʿīlīs and the Shīʿīs known pejoratively by outsiders as ghulāt ("transgressive" Shīʿīs; see Sean Anthony, Ghulāt, EI3 ) and also sometimes to Ṣūfīs, alchemists, and others. Irenaeus, author of the earliest extant major Christian heresiographical work (c. 180 C.E.), was the first to use the term gnōstikós , a