My only knowledge of Julian came from Gore Vidal's excellent book of the same name, but while he supposedly researched it very thoroughly, it's still a work of fiction.
Vidal (no apologist for Christianity) seemed to suggest Julian turned from Christianity to a pagan mystery religion because he fell under the influence of a svengali who exploited Julian's sincere belief in his religious experience. Is there any evidence of this?
If Vidal's angle is mostly fanciful, what is the true character of Julian's conversion to paganism and his stance against the church?
It was in Julian's own character. Julian was a student at the Academy in Athens. I would argue that he-like many others- was only a nominal Christian in his early life. You could say he "came out" by the time he took the role as emperor. For the rest of his life, he devoted himself to Hellenism and philosophy. Now, his stance against the church wasn't a simple religion vs religion argument. It was a little deeper than that. To put it short: he didn't view it as "Roman." Christianity contradicted--in his mind-- a lot of what Hellenism had to offer, and since "Romanness" borrowed heavily from Hellenism, the Christianity could not be "Roman." He banned the Christians from Greek teachings because of disagreement from a philosophical and social standpoint.
Also, paganism and polytheism are not the same thing. Though Julian's version of paganism harkened and paid tribute to the older gods, his words kind of say otherwise. His neoplatonic background as well as his dabblings in other eastern teachings, like Mithraism, started to develop a sort of monotheism. It's extremely confusing and conflicting. He publicly supported the old gods, but in his writings he talks as if all of them were the same. So in his battle against Christianity, he visioned a more organized paganism, one that mirrored Christianity. So where his uncle, Constantine the Great, Romanized Christianity, Julian Christianized paganism.
There are lots of interesting facets of Julian and opinion changes with each other. Vidal's book is fiction, historical, but fiction nonetheless. Go check out Rowland Smith's Julian's Gods: Religion and Philosophy in the Thought and Action of Julian the Apostate and Adrian Murdoch's The Last Pagan: Julian the Apostate and the Death of the Ancient World. There are older and more classic biographies from the 70s, but I feel more recent works would be just as good.
You might also want to look at Susanna Elm's Sons of Hellenism, Fathers of the Church: Emperor Julian, Gregory of Nazianzus and the Vision of Rome. This was not an ordinary biography. It put Julian in a dialogue with his Christian counterpart and rival, Gregory. It sort of shows the philosophical background these two men had and how it impacted Julian's Hellenism and the development of Christianity. Perhaps find it via library, these books are pricey.