Julian is called "The Apostate" because he reverted from Christianity to Paganism, and wished to roll back Constantine I's official encouragement of Christianity. Though Constantine had not made Christianity the state religion - which would not happen until Theodosius I in the 380s - he and his immediate successors had ended many of the subsidies formerly received by the pagan cults and had not discouraged the destruction of some temples. In fact, Julian's religious status is a matter of considerable dispute; he did not practice normative civic paganism of the earlier empire, but a kind of magical approach to classical philosophy sometimes identified as theurgy. Whatever his personal practices, they were not Christian.
He died in battle, and there were rumors at the time that he was killed by one of his own soldiers, a Christian who resented his beliefs. However, there was never any proof and it is now doubted.