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I am an unironic Mithraic revivalist (the Tauroctony in the Iron Pill comics was included at my request) it was basically fucking Scientology for the Roman Empire, there were seven levels of initiation that members had to go through in order to rise through the ranks. Ultimately the entire contents of the mithraic belief system have been lost to history due to it having been quite a secretive cult (some disputed fragments like the Mithras Liturgy apparently exist). It was prominent among the military and soldiers and women were banned from joining, so it has some connotations with the heroic and the vitalist. Its images, the Tauroctony, the sun banquet, the stars beneath the cape, and the leontocephaline (sometimes known as Ariamanus) are all that remains, lying around in cracked and crumbling marble waiting for new flesh and blood to interpret the symbols as they see fit.
But it's basically that the Bull represents Moloch, and he must be caught and slain.