The cult left no scripture, no liturgy and no doctrinal statement. The single line usually cited is a damaged painted inscription in the mithraeum under Santa Prisca in Rome, restored by its excavators to read as thanks for having been saved by shed blood; the restoration supplies missing letters, and even as restored it does not say what an initiate was saved from or for. Mithraists left no funerary formulae of the kind other cults of the period left in quantity, so there is nothing independent to test the reading against, and scholars working from the iconography alone have not settled it.
What would resolve this A Mithraic text of any length: a liturgical papyrus, a doctrinal fragment, or a securely legible inscription stating the cult's teaching, recovered from an excavated mithraeum or from a papyrus collection. A body of Mithraic grave inscriptions would settle it indirectly, by showing what members expected of their own deaths.
OpenRoman archaeology, Latin and Greek epigraphy, papyrologyManfred Clauss, The Roman Cult of Mithras: The God and His Mysteries