Mystery religions, mystery cults, sacred mysteries or simply mysteries, were religious schools of the Greco-Roman world for which participation was reserved to initiates "(mystai)". The main characterization of this religion is the secrecy associated with the particulars of the initiation and the ritual practice, which may not be revealed to outsiders. The most famous mysteries of Greco-Roman antiquity were the Eleusinian Mysteries, which were of considerable antiquity and predated the Greek Dark Ages. The mystery schools flourished in Late Antiquity; Julian the Apostate in the mid 4th century is known to have been initiated into three distinct mystery schools—most notably the mithraists. Due to the secret nature of the school, and because the mystery religions of Late Antiquity were persecuted by the Christian Roman Empire from the 4th century, the details of these religious practices are derived from descriptions, imagery and cross-cultural studies. Much information on the Mysteries come from Marcus Terentius Varro.