On June 26, 363, Roman Emperor Julian was killed during a retreat from the Sassanid Empire. General Jovian was proclaimed Emperor by troops on the battlefield." From the article:
"Why Julian the Apostate Failed to Revive Paganism in the Roman Empire
Julian and the Fall of Paganism
Julian the Apostate Failed to Revive Polytheism in the Roman Empire
When the Roman Emperor Julian (Flavius Claudius Julianus) came to power, Christianity was less popular than polytheism, but when Julian, a pagan (in contemporary usage) known as "the Apostate," was killed in battle, it was the end of Roman official acceptance of polytheism. Although paganism was popular, Julian's practice was more ascetic than normal pagan practices, which may be why paganism failed when the Apostate reinstated it. From Gore Vidal's Julian:
"Julian has always been something of an underground hero in Europe. His attempt to stop Christianity and revive Hellenism exerts still a romantic appeal."
When the Roman emperor Julian the Apostate, died in Persia, his supporters failed to maintain support for paganism as the official state religion. It wasn't called paganism at the time, but was known as Hellenism and is sometimes referred to Hellenistic paganism.
Instead of the ancient religion returning to the Roman Empire, the popular Emperor Constantine's Christianity re-emerged as the dominant one. This seems odd since Christianity wasn't as popular among the people as Hellenism, so scholars have searched Julian's life and administration for clues to why the apostasy (which means the "standing away from" [Christianity]) failed.
Julian (born A.D. 332), the nephew of the first Christian emperor, Constantine, was trained as a Christian, yet he is known as an apostate because when he became emperor (A.D. 360) he opposed Christianity. In The Demise of Paganism, James J. O'Donnell suggests that the emperor's particularly vehement stance against Christianity (and support for the other monotheistic religion, Judaism) stems from his Christian upbringing.
Julian's Intolerance
Although any such generalization is hazardous, pagans of the time generally held religion to be a private matter, while Christians behaved strangely in trying to convert others to their faith. They claimed that Salvation made possible through Jesus was the only true belief. In the wake of the Nicene Council, Christian leaders condemned all who failed to believe in the prescribed manner. To be a pagan in the old tradition, Julian should have let everyone worship as he or she wished. Instead of letting each person worship in his own way, Julian stripped the Christians of their privileges, powers, and rights. And he did so from their own perspective: the intolerant attitude that one's private religion is of public concern. From The Demise of Paganism:
"In summary, it is necessary to look upon the religious sociology of the fourth century with two separate (if often, and confusingly, overlapping) distinctions in mind: that between worshippers of Christ and worshippers of other gods; and that between men who could accept a plurality of worships and those who insisted on the validity of a single form of religious experience to the exclusion of all others."
Julian's Elitism
Other writers say the failure of Julian to reintegrate Hellenistic paganism into the framework of Roman society came from his inability to make it popular and his insistence that true understanding is impossible to the average mortal, but is reserved for philosophers. Another important factor was that the Christian creeds were far more unified than paganism. Paganism wasn't a single religion and adherents to different gods did not necessarily support each other.
"The panoply of religious experience in the Roman world before Constantine was simply bewildering: from back-yard fertility rites through public, state-supported cults to the mystical ascents of which Platonic philosophers wrote with such devotion—and everything between, over, under, and all around such phenomena. There were public cults indigenous to the various parts of the empire, certain generally (if often lukewarmly) accepted devotions such as that to the divinity of the emperors, and a vast array of private enthusiasms. That such a spectrum of religious experiences should produce a single-minded population capable of forming itself into a single pagan movement with which Christianity could struggle is simply not probable."
Lack of a Powerful Pagan Successor to Julian
In 363, when Julian died, he was succeeded by Jovian, a Christian, at least nominally, instead of the obvious choice, Julian's praetorian prefect, the moderate polytheist, Saturninius Secundus Salutius. Secundus Salutius didn't want the job even though it meant continuing Julian's mission. Paganism was diverse and tolerant of this diversity. Secundus Salutius didn't share the late emperor's parochial attitudes or specific beliefs.
No other pagan emperor came to power before the Roman state outlawed pagan practices. Even so 1,700 years later, we continue to be predominantly a Christian society in terms of our beliefs, it may have been the pagan attitude of religious tolerance that prevailed."