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 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 comes from Marcus Terentius Varro. Justin Martyr in the 2nd century explicitly noted and identified them as "demonic imitations" of the true faith; "the devils, in imitation of what was said by Moses, asserted that Proserpine was the daughter of Jupiter, and instigated the people to set up an image of her under the name of Kore" (First Apology). Through the 1st to 4th century, Christianity stood in direct competition for adherents with the mystery schools, insofar as the "mystery schools too were an intrinsic element of the non-Jewish horizon of the reception of the Christian message". Beginning in the third century, and especially after Constantine became emperor, components of mystery religions began to be incorporated into mainstream Christian thinking, such as is reflected by the disciplina arcani. Etymology The English word 'mystery' originally appeared as the plural Greek Mystêria, and developed into the Latin mysterium where the English term originates. The etymology of the Greek mystêrion is not entirely clear, though scholars have traditionally thought it to have derived from the Greek myo, meaning "to close or shut" (chiefly referring to shutting the eyes, hence one who shuts their eyes and is initiated into the mysteries). Some Hittite scholars suggest that the Greek term derives from the Hittite verb munnae, "to conceal, to hide, to shut out of sight". Characteristics Mystery religions formed one of three types of Hellenistic religion, the others being the imperial cult, or the ethnic religion particular to a nation or state, and the philosophic religions such as Neoplatonism. This is also