Short answers, pulled from the story.
Mithraism was a Roman mystery religion centered on the god Mithras, popular among the Imperial Roman army from the 1st to the 4th century CE. It was practiced throughout the western half of the Roman Empire, with concentrations in Rome, Ostia, Numidia, Dalmatia, Britain, and along the Rhine and Danube frontier.
The tauroctony is the central cult image of Mithraism, depicting Mithras killing a sacred bull. About 700 examples of this scene survive. Mithras is shown kneeling on the bull, stabbing it with his right hand while gripping its nostrils with his left, accompanied by a dog, snake, scorpion, raven, and two torch-bearers called Cautes and Cautopates.
Mithraism had seven grades of initiation, listed by St. Jerome. In ascending order they were: Corax (Raven), Nymphus (Bridegroom), Miles (Soldier), Leo (Lion), Perses (Persian), Heliodromus (Sun-Runner), and Pater (Father). Each grade was associated with a planet and required the initiate to undergo a specific ordeal.
Mithraists called themselves syndexioi, meaning those united by the handshake. Admission into the community was completed with a handshake with the pater, mirroring the handshake between Mithras and Sol in Mithraic mythology.
Mithraism effectively ended during the 4th century CE due to Christian persecution and the anti-pagan decrees of the emperor Theodosius in the last decade of that century. Mithraic sanctuaries were destroyed, and Beck states the religion was as good as dead quite early in the fourth century. Cumont suggested it may have survived in remote Alpine and Vosges cantons into the 5th century.
Franz Cumont argued in 1894-1900 that Mithraism was essentially Persian state religion transplanted to Rome. John R. Hinnells and R. L. Gordon challenged this at the First International Congress of Mithraic Studies in 1971, arguing the Roman mysteries were an entirely new creation. Merkelbach and Beck later proposed the cult was created in Rome by a single Greek-speaking founder from an eastern province who incorporated Iranian myths and Greek Platonism.