Outside the cavern, top left, is Sol the sun, with his flaming crown, often driving a quadriga. The Mithraeum represented the cave to which Mithras carried and then killed the bull; and where stone vaulting could not be afforded, the effect would be imitated with lath and plaster.A ray of light often reaches down to touch Mithras. They are commonly located close to springs or streams; fresh water appears to have been required for some Mithraic rituals, and a basin is often incorporated into the structure.
This indicates that the bull killing scene was used in the first part of the celebration, then the relief was turned, and the second scene was used in the second part of the celebration. In the standard pattern of Roman religious precincts, the temple building functioned as a house for the god, who was intended to be able to view through the opened doors and columnar portico, sacrificial worship being offered on an altar set in an open courtyard; potentially accessible not only to initiates of the cult, but also to colitores or non-initiated worshippers.
On the specific banquet scene on the Fiano Romano relief, one of the torchbearers points a caduceus towards the base of an altar, where flames appear to spring up. In the Suda under the entry Mithras, it states that “No one was permitted to be initiated into them (the mysteries of Mithras), until he should show himself holy and steadfast by undergoing several graduated tests.” Manfred Clauss states that the number of grades, seven, must be connected to the planets.
However, in Porphyry's Greek text De Abstinentia (Περὶ ἀποχῆς ἐμψύχων), there is a reference to the now-lost histories of the Mithraic mysteries by Euboulus and Pallas, the wording of which suggests that these authors treated the name "Mithra" as an indeclinable foreign word. Beck argues that religious celebrations on this date are indicative of special significance being given to the Summer solstice; but this time of the year coincides with ancient recognition of the solar maximum at midsummer, whilst iconographically identical holidays such as Litha, St John's Eve, and Jāņi are observed also.
Modern historians have different conceptions about whether these names refer to the same god or not. Hinnells has written of Mitra / Mithra / Mithras as a single deity worshipped in several different religions. For their feasts, Mithraic initiates reclined on stone benches arranged along the longer sides of the Mithraeum – typically there might be room for 15 to 30 diners, but very rarely many more than 40 men.
(See section Interpretations of the bull-slaying scene below.) The practice of depicting the god slaying a bull seems to be specific to Roman Mithraism. In some Mithraea, such as that at Dura Europos, wall paintings depict prophets carrying scrolls, but no named Mithraic sages are known, nor does any reference give the title of any Mithraic scripture or teaching.