Harold R. Willoughby's 1929 book, Pagan Regeneration, explores various Greco-Roman mystery cults, analyzing their rituals and beliefs. The author examines the significance of these cults in satisfying the spiritual and emotional needs of individuals during the Roman Empire. Willoughby focuses on several key cults, including the Eleusinian Mysteries, the worship of Dionysus, Orphism, the cult of Cybele and Attis, Mithraism, and the Isis cult. He details their respective rituals, myths, and theologies, highlighting their common themes of rebirth, salvation, and mystical union with the divine. The book ultimately argues that these pagan cults played a crucial role in shaping religious and philosophical thought leading up to the rise of Christianity.