Flavius Claudius Julianus was a son of Julius Constantius (half-brother of Constantine the Great) and his 2nd wife, Basilina. He was, therefore, a nephew of Constantine the Great, who famously decriminalised Christianity in 313 CE, thereby initiating the development of the Roman empire into a Christian one. Basilina died soon after Julian was born and his father was killed in 337, a death arranged by the 3rd son of Constantine the Great who became Constantius II and who ruled the empire from 337 to 361 (and who sounds like a right bastard).

With his older half-brother, Gallus, Julian was apparently cared for first by his maternal grandmother and then by Eusebius, a scholar and (arguably) historian of Christianity who became the bishop of Caesarea Maritima in around 314.

Later, Julian (again with Gallus) lived at Macellum, a remote estate in Cappadocia.

At some point, Julian was summoned to Milan by Constantius II’s second wife, Eusebia, who ensured the scholarly lad was given books – philosophy, poetry, history – and sent out to be in charge of the Roman empire’s “western army in Gaul” (76).

When he was near death, Constantius II declared Julian to be his successor.

Thus, Julian became emperor in December 361, at which point he declared himself to be a pagan (although he’d rejected Christianity much earlier), hence he became Julian the Apostate amongst Christians. Julian wrote a number of works in Greek, some of which have subsequently been edited and translated. There are questions about the authenticity of the some writings, and concerns that hostile Christians may have suppressed others.