Ostia, Oratorio Cristiano delle Terme di Mitra is a late 4th or early 5th century ruined church built over the Mithraeum at the Baths of Mithras in the Via della Foce, which is in the north-west quarter of the ancient ruins of Ostia and near the Tiber.
The Mithraeum under the baths, after which they were named, apparently suffered a hostile takeover by Christians after the suppression of pagan places of worship by the emperor Theodosius I in 395. Fragments of the smashed statue of Mithras, found in a drain by archaeologists, gave witness to the violence. In the conversion, a small oratory was furnished from one of the rooms of the bath complex (presently referred to as Room G).
No documentary evidence survives for the oratory, so its dedication and ecclesial function remain unknowable.
What survives of this oratory is the low brick wall of the segmental apse, flanked by two free-standing marble columns in situ. The wall is opus vittatum, meaning that the brick courses are separated by tiles.
Four small square marble columns were found here on excavation, two of which have the chi-rho symbol carved on the top of one face. Their side faces have grooves, which indicate that they formed part of a stone screen or iconostasis. They are 1.3m high.