Do some research on the real-world religion of Mithras, and the Persion Mithra and you will probably find out everything you are looking to know. Wikipedia has an excellent article on the religion.
Deriving information from that ancient god, one can reasonably say that the Mitra religion is a mystery religion. Some of the symbols include a bull.
Here is soemthing interesting from Wikipedia: The members of a mitraeum are divided into seven ranks. All members are expected to progress through the first four ranks, while only a few would go on to the three higher ranks. The first four ranks represent spiritual progress, while the other three are specialized offices. The seven ranks are:
Corax (raven)
Nymphus (bride)
Miles (soldier)
Leo (lion)
Perses (Persian)
Heliodromus (sun-courier)
Pater (father)
The new initiate became a Corax, while the Leo was an adept. The titles of the first four ranks suggest the possibility that advancement through the ranks was based on introspection and spiritual growth.
Mitra died, was buried in a rock tomb, and resurrected, so a symbol may well be a cave or rock tomb. When Mitra killed the sacred bull, the entire world sprang into existance, thus the bull symbol is appropriate. The Lion and the Cup are another symbol.
The religion is a mystery religion with a life-death-rebirth deity, comparable to Isis, the resurrected Jesus or the Persephone/Demeter cult of the Eleusinian Mysteries if you feel like doing more research.