Every mithraeum had the same image at its focal end. Mithras, a young man in a Phrygian cap and streaming cloak, kneels on a bull and drives a dagger into its shoulder while turning his face away. A dog and a snake go for the wound, a scorpion attacks the bull's genitals, a raven perches nearby, and ears of grain sometimes grow from the bull's tail or from the blood. Two torchbearers flank the scene, Cautes with his torch raised and Cautopates with his lowered. Sol and Luna occupy the upper corners. This is the tauroctony, and it is reproduced with only small variation from Britain to Syria.
Many reliefs are carved on both faces or set to pivot, and the reverse shows Mithras and Sol at a banquet on the bull's hide. Congregations ate together on benches running down the long sides of the room, which suggests the banquet, and not only the killing, was what members re-enacted.
Initiation ran through seven grades. They are listed by the Christian writer Jerome and laid out in a mosaic floor at the Mithraeum of Felicissimus at Ostia: Corax the raven, Nymphus the bridegroom, Miles the soldier, Leo the lion, Perses the Persian, Heliodromus the sun-runner, and Pater the father. Each carries a planetary association. What a grade actually required is known only from fragmentary paintings, scratched graffiti and a hostile Christian description of ordeals.
Astronomical symbolism is unmistakable. Zodiacs frame many reliefs, and every figure in the tauroctony matches a constellation lying along a continuous band of sky. What that symbolism meant to an initiate is a separate question, and a harder one.