Wikipedia fails to summarize the world we get in this novel, saying that Julian primarily concerns itself with “the changes to Christianity wrought by Constantine and his successors”, and though this is true I think it’s only the beginning of the bigger ideas presented in the book. The Emperor Julian is a devout pagan but everyone else in the novel thinks his beliefs are outdated, simplistic, immature, and absurd; every other religious figure in the text is a manipulative liar, and utilizes faith only as a method of personal advancement. Paganism and Christianity are both either superstitions or means to achieve greater and greater power.
(I am a wikiphile, but the article is also wrong when it says that Gore Vidal himself writes in the very short introduction that the novel “deals” with the changes wrought to Christianity by Julian—Gore Vidal summarizes Julian’s life and mentions this fact, but nowhere says that his novel is principally about these changes)
The next level of ideas goes beyond that—virtually every text that deals with Rome must somehow examine the question of Why Did Rome Fall?, even though the corrupt demagoguery of Republican Rome is still thriving in American politics and even if practically every conqueror of Rome styled himself as a Roman Emperor—and the Emperor Julian himself has this question on his mind (as does Marguerite Yourcenar’s Hadrian), both of whom notice and complain that the intellectual vigor of previous generations has been lost, that there are no new ideas and that everyone looked upon as wise simply quotes the ancients and memorizes as much of Homer as humanly possible. Julian, his two philosophers, and Hadrian (if memory serves…) all say that this is why Rome is falling apart.
But I think Vidal’s answer to this great silly question is different: before Julian becomes Emperor everyone must constantly watch his or her mouth, afraid of all the informers and spies who seem to be the Empire’s only citizens; one cannot badmouth an Emperor, even though there may be multiple rival Emperors at any given time, and the victory of one over the other is usually a matter of chance. Then, after Julian’s accession, Gore Vidal establishes with one sentence that justice at this time is absolutely no different from whoever happens to be in power—
That evening [Eusebius] was arrested for high treason and sent to Chalcedon to stand trial.
This character, Eusebius, was in power for decades, and was the be all and end all of right and wrong—Julian did away with him in a heartbeat. If people cannot speak their minds, and if justice is so arbitrary, a decline in intellectual vigor is obvious and inevitable.
So, postscript: a better question to ask is Why Didn’t Rome Fall? Why is that snake so swollen with life, so vicious, and so vigorous, today?