The author of the 'Refutation of All Heresies', competing bishop of Rome (ca. 225 CE), wrote perhaps the largest and most thoroughgoing attack on his (mainly Christian) opponents. His sensational thesis--that his enemies plagiarized their arguments from Greek philosophy, magic, astrology, and mystery cults--still inspires fascination today. In elucidating his thesis, the author uniquely preserves a veritable library of ancient philosophy, astrology, medicine, magic, gnostic thought, and numerology--in addition to offering some lurid sidelights on the ecclesial politicians of his day. Offered here for the first time in almost a century is a full length English translation, along with a newly edited Greek text and extensive notes. A thorough introduction addresses questions of authorship, date, and audience, as well as the book's purpose, organization, method, and importance for gnostic studies.