Mithraism was a philosophical and religious movement believed to have originated in the ancient Middle East of Old Earth late in the Hellenistic period, about the time the Roman Empire conquered the area.
It gained prestige by claiming connection with a contemporaneous change in the Zodiacal arrangement in the sky visible from Earth. Its followers were known as Mithrans. It was in some respects, one of the mystery cults, its tenets and practices becoming obscure after it died out. It was believed that some of these practices were adopted by Christianity.
The religion fell into decline, more or less with the Roman Empire, and had completely disappeared by the 16th Century Ante Diaspora. It was revived, as much as possible given its obscurity, in the following millennia and the revived version came to be widely practiced throughout the worlds of the Diaspora, including the Star Kingdom of Manticore.