From the 2nd century onwards, Eastern mystery cults, centred on a single deity (though not necessarily monotheistic) and based on sacred truths revealed only to the initiated, spread widely in the empire, as polytheism underwent a gradual, and ultimately terminal, decline. One such cult, that of Sol Invictus ("The Invincible Sun"), was designated as the official army-cult by the emperor Aurelian (r. 270–5) and remained such until the time of Constantine I (r. 312–37). However, by far the most popular among the Roman military was Mithraism, centred on a deity called Mithras. The mainstream view is that this originated in the Persian cult of Mithra, but the salient features of the Roman cult are absent in the Avesta and other Iranian evidence. It is thus possible that the Roman cult was not connected to the Iranian (except perhaps that the deity's name was borrowed) and instead originated in the eastern provinces of the empire itself, most likely in Phrygia.cf. Plutarch Pompey 24 Mithraism was probably a medley of elements from various cults – hence its apparent adoption of a Persian deity-name, of the taurobolium ritual from the cult of Cybele, and of the Phrygian cap. Based on secret initiation ceremonies and rites, this cult is attested, for example, by the discovery of a Mithraeum (Mithraic temple) at Carrawburgh fort near Hadrian's Wall. Membership, according to the written evidence of dedications in Nida (Heddernheim), was not restricted according to social standing.Meier-Arendt Römische Steindenkmäler aus Frankfurt am Main, Museum für Vor- und Frühgeschichte Frankfurt, Archäologische Reihe 1(1983)
