Short answers, pulled from the story.
Julian was the Roman emperor from 361 to 363, born around 331 as a nephew of Constantine the Great. He was called the Apostate in the Christian tradition because he rejected Christianity and actively promoted Neoplatonic Hellenism as emperor, attempting to restore traditional Roman paganism as the empire's primary religion.
Julian was appointed Caesar of the West in 355 by his cousin Constantius II and sent to govern Gaul. In February 360, his troops proclaimed him Augustus in Lutetia (Paris) after Constantius ordered Julian's Gallic soldiers to the eastern front. Civil war was avoided when Constantius died on the 3rd of November 361, having named Julian his successor; Julian entered Constantinople as sole emperor on the 11th of December 361.
Julian departed from Antioch on the 5th of March 363 with a force estimated between 65,000 and 95,000 men. The Romans won a tactical victory outside Ctesiphon but did not besiege the city, then Julian ordered the fleet destroyed and marched into the Persian interior. Facing supply shortages and Persian harassment, the army retreated northward; during the Battle of Samarra on the 26th of June 363, Julian was struck by a spear and died three days later.
Julian stripped Christian bishops of the stipends and legal privileges Constantine had granted them, restored confiscated pagan temples, and issued a freedom of religion edict on the 4th of February 362. His School Edict barred Christians from teaching classical texts. He also tried to reorganize the pagan priesthood along more structured, charitable lines and authorized the rebuilding of the Jewish Temple in Jerusalem in 363.
Julian practiced Neoplatonic paganism, viewing traditional myths as allegories in which ancient gods represented aspects of a philosophical divinity. He learned theurgy from Maximus of Ephesus and was influenced by the system of Iamblichus. His chief surviving religious works are To King Helios and To the Mother of the Gods. He was also initiated into the Eleusinian Mysteries during his time in Athens.
Julian wrote extensively in Greek across his reign, including panegyrics, philosophical essays, satires, and letters. Surviving works include the Misopogon (a satire on his conflict with the people of Antioch), The Caesars (a satirical contest among Roman emperors), To King Helios, To the Mother of the Gods, and his Letter to the Senate and People of Athens. His polemic Against the Galileans survives only in fragments preserved by Cyril of Alexandria.