The Heresy of Jacob Frank is the first monograph length study on the religious philosophy of Jacob Frank (1726-1791), who, in the wake of false messiah Sabbetai Zevi, led the largest mass apostasy in Jewish history. Based on close readings of Frank's late teachings, recorded in 1784 and 1790, this book challenges scholarly presentations of Frank that depict him as a sex-crazed "degenerate," and presents Frank as an original and prescient figure at the crossroads of tradition and modernity, reason and magic, Kabbalah and Western Esotericism. Frank's worldview combines a skeptical rejection of religious law as ineffectual and repressive with a supernatural, esoteric myth of immortal beings, material magic, and worldly power. With close readings of the theological and narrative passages of Frank's teachings, Michaelson shows how the Frankist sect evolved from its Sabbatean roots and the infamous 1757-59 disputations before the Catholic Church, into a Western Esoteric society based on alchemy, secrecy, and sexual liberation. Sexual ritual, apparently tightly limited and controlled by the sect, was not a libertine bacchanal but an enactment of the messianic reality, a corporealization of what would later become known as spirituality. While Frank was undoubtedly a manipulative, even abusive leader whose sect mostly disappeared from history, Michaelson suggests that his ideology anticipated themes that would become predominant in the Haskalah, Early Hasidism, and even contemporary 'New Age' Judaism. In an inversion of traditional religious values, Frank's antinomian theology held personal flourishing to be a religious virtue, affirmed only the material, and transferred messianic eros into social, sexual, and political reality. Michaelson reconstructs Frank's teachings with critical methodology, tracing how Frank both followed and resisted the disciplines of reason, magic, Kabbalah, and esotericism.