It is debatable what his vision of a return to religion of the Greeks and Romans really entailed, whether an embracing of the philosophy of Neoplatonism or a true devotion to Greek mythology and the pantheon of Mount Olympus and its cult.
Nonetheless, his passion was for a return to a non-Christian Greco-Roman culture, and he was as passionate about religion opposed to Christianity as the Christians were about their converting the world. All else—taxes, communication, empire—were secondary to both groups.
He grew up in a world of political rivalry and blood-letting among the family members of Constantine the Great. Because he was only a small boy, his life was spared when Constantine’s illegitimate descendants killed his father and all of his uncles in 337 c.e.
He was shuffled off here and there by wary imperial wardens, first to bustling Nicomedia and then to lonely, distant Cappadocia. In such an isolated existence he discovered the world of the Homeric epics and Hesiod, the twin sources of the Greek pagan "Bible" canon.
He also found consolation in the beauty of nature and thus was prone to sympathy for the pantheistic theology of his Greek forebears. Later, as a teenager, he was exposed to the philosophy of some excellent Neoplatonic teachers (Libanius, Maximus, and Iamblichus).
All of these factors—the strife that Christianity brought to his childhood, the stability of the Greek classics, his sensitivity, and the explanation of philosophy—led Julian to undergo a secret "conversion" to Greek (pagan) religion by the age of 20. He was initiated into the mystery of Mithras at that time.
His true colors did not show up publicly for at least 10 more years. At the age of 24 he was summoned by his cousin Emperor Constantius II to do service in the West. Although he was named Caesar, he was closely supervised by the emperor’s key personnel. He had no particular military training, yet he acquitted himself well on the battlefield against the Germans over the next five years.
At that point the emperor ordered Julian and his troops to go east and fight against the Persian Sassanid Empire. Instead, after a careful conspiracy his troops mutinied and proclaimed Julian Augustus, that is, emperor, in place of Constantius II.
A civil war was avoided only because his cousin suddenly died in 361. At the age of 30 Julian was sole leader of the Roman Empire and could with impunity proclaim that he was not a Christian but a believer in Greek religion. He purged the bloated imperial bureaucracy for the sake of effociency and to eliminate Christian influence.
He showed youthful energy in taking actions to reinstate paganism: He cut off funding for churches; he refurbished Greek temples and tried to reinstate the Greek priests and bloody sacrifice; and he forbade Christians from teaching the Greek classics.
He tried to undermine Christianity in several ways. One of his quirky ideas was to rebuild the Jewish Temple in Jerusalem, a project probably intended as a jab at the Christians. That idea was abandoned when an earthquake occurred just before reconstruction was to begin.
Another, more reasonable idea was to proclaim complete religious tolerance, probably in hopes that Christianity would disintegrate into a morass of doctrinal wrangling. He exiled one of Christianity’s venerable heroes, Athanasius, and he refused to rescue the Christian intellectual center, Nisibis, when the Persians captured it.
By the same token he wanted to prove that he was both serious about his duties as emperor and endowed with the blessings of Zeus and the gods, so he undertook a campaign against the Persians.
He was ingloriously defeated and died in 363 in the dry lands of Mesopotamia, less than two years after he had assumed power. His program had generated only limited public support, and in the end the empire reverted into the hands of the Christians, determined never again to let another such pagan take over.