The book surveys the development of secret doctrines and ritual orders from ancient Near Eastern and Egyptian priesthoods through Hellenic and Roman mysteries, including Eleusinian and Dionysian rites, and the philosophical sects such as Pythagoreans and Orphics. It examines Jewish ascetic communities and early Christian origins, profiles alleged miracle-workers and medieval institutions like the Knights Templar and stonemasons' lodges, and traces the emergence of freemasonry, Rosicrucianism, the Illuminati, and various eighteenth- and nineteenth-century clandestine movements. Throughout, it emphasizes recurring themes of initiation, symbolism, social function, and the persistent human attraction to secrecy.