Flavius Claudius Julianus (331 - 363 AD) was Roman emperor between 361-63 AD. Part of the Constantinian Dynasty, he was the nephew of Constantine I; and successor of Constantius II.
Early on he pursued the life of a scholar studying Christianity among other things, but was forced into public service upon the death of his brother Gallus and made Caesar (heir to the throne). He was a successful general, fighting the Franks and Alemanni in Gaul and later against the Persians. Rivalry between him and the Emperor Constantius eventually forced a near civil war.
Constantius died before the two could meet in combat to settle the issue and Julian was proclaimed the sole emperor. At some point Julian abandoned his Christian roots and tried to re-establish paganism as the main religion of the empire. While still supporting religious tolerance, Julian was later vilified by the church for his actions, though he was actually an excellent administrator in his short reign. Julian was an accomplished writer and his works mainly debating the merits of Christianity and Paganism are an invaluable resource into the ancient faiths.
Works:(available in modern collections generally titled the "Works of Julian")
Epistula ad SPQ Atheniarum