Nonnus Abbas on Gregory of Nazianzus

Mithra therefore the Persians consider to be the sun, do sacrifice to him, and observe certain rites in his honour. No one can participate in his service without passing first through the grades of discipline. These grades are eighty in number, with descent and ascent, for the tests applied are first of an easier character, then more difficult; and thus after passing through all the grades the disciple arrives at perfection. The successive disciplinary tests are by fire, by cold, by hunger and thirst, by prolonged exertion, and in a word by similar trials of all kinds.

Commentary on Gregory Nazianzen, In Julianum imperatorem invectivæ duæ

Mithra is considered by the Persians to be the sun. And to him they offer many sacrifices, and observe certain rites in his honour. No one can be initiated into the rites of Mithra without passing through all the disciplines and giving proof of self-control and chastity. Eighty grades are enumerated through which the postulant must pass in succession; for example, plunging first into deep water for many days, then throwing himself into fire, then solitary fasting in a desert place, and others also until as stated above he has passed through the eighty. Then finally if he survives he receives the highest initiation, or if he has succumbed an (honourable) sepulture.

Commentary on Gregory Nazianzen, In Julianum imperatorem invectivae duae

Different views are held with regard to Mithra. Some identify him with the sun, others with the guardian of the fire, others with a specific force, and certain rites are observed in his honour, especially among the Chaldaeans. The aspirants to initiation pass through a series of disciplinary grades, undergoing first the easier forms of penance, then the more difficult. For example fasting is first imposed upon the neophytes for a period of about fifty days. If this is successfully endured, for two days they are exposed to extreme heat, then again plunged into snow for twenty days. And thus the severity of the discipline is gradually increased, and if the postulant shows himself capable of endurance he is finally admitted to the highest grades.

Commentary on Gregory Nazianzen, In sancta lumina

All translations by A. S. Geden