Julian the Apostate was emperor of Rome from 361-363 CE and the nephew of Constantine. Raised in a strict Christian environment (although of the Arian tradition), he formally announced his conversion to paganism in 361 and became a public enemy of Christianity.
That provides the background for Vidal's excellent historical novel (historical in the best sense in that Vidal tried to use as many actual events and recorded conversations as possible). Vidal is, of course, rather flagrant in rejecting Christianity himself, so it is easy to see why Julian's gradual rejection of what he viewed as a faith filled with contradictions both in belief and behavior would be appealing to Vidal.
The book is told from Julian's point of view as a form of autobiography with little side social commentaries of two of his friends. The debate between the supporters of Athanasius (who finally won out) and the Arians is well explained. In the fourth century (see also When Jesus Became God, the debate over the divinity of Jesus was of huge consequence. The Arians (basing their case on John 14:25) believed in the doctrine of homoiousios: Jesus was a similar substance to God the father but created by him. The followers of Athanasius adopted that "pernicious doctrine" later codified in the Nicene Creed of homoousius (meaning that God, Jesus, and the Holy Spirit are one and the same).
It was essential for Julian to pretend to be one of the Galileans, as Christians were called then, because it was the declared religion of Rome after Constantine. As a potential successor to the throne, he was subject to all sorts of plots and political machinations, and these dangers form much of the tension of the book, as Julian tries to remain alive posing as a student of philosophy with no interest in politics. Julian's childhood was that of a prince with all that entails, including constant supervision, little access to people besides his siblings, and strict regulation of behavior. Julian's cousin, the reigning emperor Constantius, fearing for his throne, systematically murdered those who might be a threat -- especially his relatives -- so Julian had to tread very carefully. Fortunately, Julian was needed to be the titular head of Gaul, so he was removed from Athens, married to Constantius's sister, Helena, and sent to barbarian Europe. Julian, whom the emperor suspected had no military prowess, surprised everyone with his skill in battle as well as administratively, even though his hands were often tied by Constantius's Florentius, who had a great deal of administrative control. Constantius's attempts to subdue the Persians was to prove his undoing, and when he demanded that virtually all of Julian's troops be sent to him - despite Julian's promise to the troops from Gaul that they would not have to serve outside the province - those troops rebelled and demanded that Julian be appointed Julian Augustus, i.e., Emperor of the West. Helena, by this time, even though she was sister to Constantius, sided with Julian, because she knew that her brother had murdered her two children because he feared them as threats to his throne. Before a civil could result Constantius died.
Julian's (Vidal's?) comments on power and the corrupting role of imperialism are as pertinent today as they might have been two centuries ago: " Wherever there is a throne, one may observe in rich detail every folly and wickedness of which man is capable, enameled with manners and gilded with hypocrisy." "I have often felt when studying history that not enough is made of those intermediaries who so often do the actual governing. . . As a result, factions within the court could form and reform, irrelevant to the nominal power. . . .On the throne of the world, any delusion can become fact." The corruption and greed become palpable in Vidal's words.
Vidal uses a triple narrative technique that intersperses Julian's "autobiography" with comments by two contemporaries, a philosopher and a rhetorician, whose views do not always coincide with Julian's, permitting Vidal to offer disparate views of events. Julian is ultimately portrayed as a pagan philosopher-leader struggling against the hypocrisy of the new Galilean religion and trying to recapture the glory of the lost Hellenistic past.
Julian used his military and imperial rights to revive paganism and subdue the upstart Christian cult, but was killed - Vidal suggests by one of his own men, perhaps at the direction of the bishops - during the war against the Persians.
Vidal has vividly captured the intense political maneuvering and danger of being in line to succeed to the throne. This is historical fiction at its nail-biting best.