The religion and its followers faced persecution in the 4th century from Christianization, and Mithraism came to an end at some point between its last decade and the 5th century. Ulansey states that “Mithraism declined with the rise to power of Christianity, until the beginning of the fifth century, when Christianity became strong enough to exterminate by force rival religions such as Mithraism.” According to Speidel, Christians fought fiercely with this feared enemy and suppressed it during the late 4th century. Mithraic sanctuaries were destroyed and religion was no longer a matter of personal choice.[ae] According to Luther H. Martin, Roman Mithraism came to an end with the anti-pagan decrees of the Christian emperor Theodosius during the last decade of the 4th century.
Clauss states that inscriptions show Mithras as one of the cults listed on inscriptions by Roman senators who had not converted to Christianity, as part of the “pagan revival” among the elite in the second half of the 4th century.[af] Beck states that “Quite early in the [fourth] century the religion was as good as dead throughout the empire.” Archaeological evidence indicates the continuance of the cult of Mithras up until the end of the 4th century. In particular, large numbers of votive coins deposited by worshippers have been recovered at the Mithraeum at Pons Sarravi (Sarrebourg) in Gallia Belgica, in a series that runs from Gallienus (r. 253–268) to Theodosius I (r. 379–395). These were scattered over the floor when the mithraeum was destroyed, as Christians apparently regarded the coins as polluted; therefore, providing reliable dates for the functioning of the mithraeum up until near the end of the century.: 31–32link
There is a theory that elite Mithraists became elite Christians who founded and maintained the Catholic Church. The rapid decline and disappearance of Mithraism parallels the rise of Christianity. Since it is today now clear, in my view, that Christianity is not going to be able to guide the West forward, and indeed will impede its development, I think we should consider the possibility that Christianity never was what people thought it was. That what it really was was an elite cult of mind-control and subjugation of the West. ABN
“Mithraism had backing from the Roman aristocracy during a time when their conservative values were seen as under attack during the rising tides of Christianity.” ~ link. If so, this provides a good reason for the Roman aristocracy to quietly switch over from Mithraism to Christianity thus retaining or even increasing their control of Roman culture. ABN