The mystery cult of Mithras must have been very popular in Ostia. The city had at least 17 mithraea or places of worship. It should, however, be noted that these mithraea were not that large: they could perhaps accommodate twenty to thirty people each. Twenty to thirty men actually, as initiates of the cult were exclusively male. The cult of Mithras was especially popular among soldiers, which explains why many mithraea were found along the borders of the Roman Empire. But the number of uncovered mithraea in multicultural, cosmopolitan cities such as Rome and Ostia is significant as well. Apart from soldiers, low-ranking officials, freedmen and slaves also joined the cult as initiates. Even the emperor Commodus is said to have participated in the rites, although according to the gossipy Historia Augusta he committed an act of sacrilege by murdering someone during the solemnities. In spite of its popularity, Mithraism never became a mass movement, nor a serious rival to Christianity. The cult was at its peak in the second and third century, but then quickly went into decline. Christian churches were often built over abandoned mithraea, and good examples in Rome are the churches of San Clemente and Santo Stefano Rotondo.
Mithraeum of Felicissimus
Of the 17 mithraea in Ostia, at least five can be visited. By far the most interesting one is the mithraeum of Felicissimus. It presumably dates from the third century and was named after the man who had it built (or who commissioned the floor), apparently in fulfilment of a vow (EX VOTO). The mosaic floor of the mithraeum has been preserved, and it provides us with a lot of information about the grades of initiation that were known in Mithraism. Seven of these grades have been depicted. The mosaics can be interpreted in the light of a letter that the church father Saint Jerome of Stridon (ca. 347-420) wrote to a certain Laeta at the beginning of the fifth century. The letter is mostly about the way Laeta’s daughter should be raised, but on a sidenote Jerome also writes about a relative of Laeta, one Gracchus, who had been urban prefect of Rome. The church father writes that this Gracchus, as praefectus urbi, destroyed a cave of Mithras (specum Mithrae), thereby also obliterating images of the grades of initiation. Jerome subsequently mentions the grades of Corax (Raven), Nymph(i)us (Groom), Miles (Soldier), Leo (Lion), Perses (Persian), Heliodromus (Sun-runner) and Pater (Father). The destruction of the mithraeum must have taken place in 376-377, as a Gracchus is known as city prefect for those years.
Each of the seven grades is associated with a planet. The Raven is accompanied by a caduceus, the staff of Mercury, who is both a deity and a planet. The image of the groom is unfortunately quite damaged on the left side: we only see a lamp and diadem. However, we know the relevant planet is Venus. The Soldier obviously has a connection with Mars, the Lion with Jupiter and the Persian with the Moon (in the Roman world, the Moon was considered a planet). The Sun-runner is associated with the Sun, which the Romans also considered a planet. Lastly, the Father is associated with Saturn. Readers can find more information about the grade-planet associations here.Other mithraea
Not far from the mithraeum of Felicissimus we find the mithraeum of the Snakes from the middle of the third century. It is named after the frescoes in the building that feature two snakes. Unfortunately the floor of the building is completely gone; it may have been made of wood. On the other hand, the benches on which the participants in the cult must have sat are still there. At the far end of the mithraeum are the remains of what must once have been the altar. Interestingly, the wall frescoes are older than the mithraeum itself. They were originally part of an older sanctuary and were probably preserved because the snake plays and important role in Mithraism as well. When Mithras kills the bull – the tauroctony – he is aided by several animals, including a snake.
The mithraeum of the Planta Pedis is in a somewhat remote corner in the west of Ostia. It has a simple mosaic floor featuring a snake. The snake is at least four times the size of the mosaic of a footprint after which the mithraeum takes its name. This explains why during my last visit to Ostia I did spot the snake, but not the footprint. The footprint was of major religious importance. It represented the foot of Mithras himself, and the participants in the cult used to place their own foot on it. Against the back wall the mithraeum must have had a marble relief depicting Mithras killing the bull. Of the relief only fragments of the Sun (Sol) and Moon (Luna) have been preserved. These have, by the way, been replaced with plaster copies. Mithras is always called Sol Invictus Mithras in inscriptions. It follows that he is associated with the sun god Sol, but at the same time he is a separate individual, and Mithras and Sol are routinely depicted separately. It is possible that initiates of Mithras tried to link their cult to the official cult of Sol Invictus. After all, the latter cult was a state cult in the second and third century. As such, it was universally accepted, unlike Mithraism, a mystery cult which was always mistrusted a bit.
A beautiful statue from the second century was recovered from the mithraeum of the Baths of Mithras. It represents the hero killing the bull. According to an inscription the statue was made by a certain Kriton from Athens, and it can currently be found in the museum of Ostia. Archaeologists presume that it had a couple of metal parts, in any case the blade of Mithras’ knife and the sun rays emanating from his head.
I must have completely missed the mithraeum of the Seven Spheres. It is located just west of the theatre of Ostia, but I think visitors can only peep through the bars of the door. The mithraeum is especially famous for its mosaics, among them images of Cautes and Cautopates, Mithras’ aides. We also find mosaics of six planets, twelve signs of the zodiac and the knife of Mithras here. The name of the mithraeum derives from the seven arches or heavenly spheres visible on the mosaic floor.
The five mithraea discussed here were all mentioned on a plan of Ostia that I was given during my visit in the summer of 2018. All sanctuaries of Mithras can be found on the Ostia Antica website, although I do not think that all of them can be visited.
 The Cambridge Dictionary of Classical Civilization 2006/2008, p. 585.
 Historia Augusta, Commodus Antoninus 9.