he's simply using it as a synonym for mystery schools, which are varied, and most of them have gnosticism at its core. basically a mystery school is one in which the knwoledge, or gnosis, is tightly guarded and only told to initiates, and only then in stages, called degrees. the cult of mithras had 7 degrees, much like the catholic church has 7 sacraments. there is also a statue of mithras slaying the bull at the vatican to go along with the obelisks. this has its origin in astrotheology, in which the stories of the gods are used to explain the movements of the sun, moon, stars and planets. so here mithras is the age of mars, the warrior, and the bull is the passing age of taurus. this is also the ending to apocalypse now btw.
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