The second half of the fourth century was decisive for the outcome of the struggle between Christianity and paganism. The unwillingness of the Emperor Julian to conform to his rigid Christian upbringing led to his being named the Apostate. Strongly under the influence of the neo-platonic school, with an inclination towards the mystical, Julian declared himself a convinced Mithraist—and we should stress the word convinced, for the fourth century produced many sympathisers, but few true followers of Mithras.
J. Bidez, who has written a fine biography of Julian, describes him in glowing terms as the last emperor who professed the Mithraic faith. Julian recognised that if Mithraism were to become the world religion, it had to discard many of its more primitive aspects and be prepared to assimilate more philosophical elements, a consideration which must have contributed to those signs of the mysticism of Iamblichus which appear in the Emperor's own Hymn to the Sun. Mithras is the Sun and is one and the same with Apollo, phaethon, Hyperion and Prometheus. The other gods merely express different aspects of the power of the sun.
Julian saw himself in the role of a good shephered, whose moral code was laid down by Mithras: 'Goodness towards the people he had to rule, piety towards the gods and moderation'. From the moment that he was initiated in a Mithraeum at Constantinople and entered into the highest grade of the cult he did everything in his power to encourage the triumph of the Mithraic cult, but his life was cut short by an arrow during his expedition against the Persian King Shapur.
After his death in A.D. 363 a period of comparative tolerance set in, but this was cut short by an edict of the Emperor Gratian in A.D. 382. The altar of Victory was removed from the Senate, and state support for the upkeep of the Roman cult was withdrawn. Gratian was in A.D. 379 the first emperor to refuse the high dignity and title of pontifex maximus. Shortly before this (A.D. 377) the city perfect Gracchus had, according to Hieronymus, overturned, broken and destroyed a cave of Mithras filled with monstrous images. We do not know exactly which Mithraic temple this was; de Rossi though it might be the sanctuary at San Silvestro. Be that as it may, the traces of such an iconoclastic act are clearly visible in the temple of Santa Prisca.
Gratian found himself in opposition to a group of prominent intellectuals. These can be divided into two groups, one of which wished to follow the example of Julian and the other to support the gods whose existence, according to Altheim, was founded 'not in their being gods, but in their being gods of Rome'. Both groups, however, worked closely together against Gratian. Their leaders included Vettius Agorius Praetextatus who restored the Porticus Deorum Consentium with the statues of the twelve gods at the Forum. He occupied various priestly offices and was Father of Fathers in the cult of Mithras. Praetextatus was a faithful follower of Julian's ideas, while his successor and friend Q. Aurelius Symmachus was a staunch conservative.
Verius Nicomachus Flavianus, a cousin of Symmachus, who was later to carry on the final struggle, was punished by the emperor in A.D. 377 because of his support of the Donatists in Africa. Various important inscriptions by Alfenius Ceionius Julianus Kamenius, a cousin of the Emperor Julian, show his faith in Mithras. Included in this circle of aristocrats and scholars was the author Macrobius who referred to the doctrine of the pagan world in his Saturnalia.
Symmachus, an able diplomatist, took upon himself the thankless task of remonstrating with Gratian about his decision. Whereupon Ambrose, bishop of Milan, threatened the young Emperor with excommunication. The protagonists, however, were not Gratian and Symmachus but their respective associates Ambrose and Praetextatus, and when the latter died in A.D. 385 he left his party without a leader.
—Vermaseren (1963) Mithras, the Secret God
Mithras in his work
'As for thee', Hermes said to me [Julian], 'I have granted you the knowledge of thy father Mithras. Do thou keep his commandments, and thus secure for thyself a cable and sure anchorage throughout thy life, and when thou must depart from the world that canst with good hopes adopt him as thy guardian god.'
In Rome maiden priestesses guard the undying flame of the sun at different hours in turn; they guard the fire that is produced on earth by the agency of the god. And I can tell you a still greater proof of the power of this god, which is the work of that most divine king himself. The months are reckoned from the moon by, one may say, all other peoples; but we and the Egyptians alone reckon the days of every year according to the movements of the sun. If after this I should say that we also worship Mithras, and celebrate games in honour of Helios every four years, I shall be speaking of customs that are somewhat recent.
The inscription explains the transmission of the fourth Mithraic degree through the Paters of the Mitraeum of San Silvestro.
Emperor Julian may have been initiated into the cult of the god Mithras at the Mithraeum of Vienne, France, according to Turcan.
This monument depicts Mihr/Mithras watching over the transition of power from Shapur II to Ardashit II, which took place in 379.