Julian the Apostate
Upon becoming emperor, Julian decided to restore paganism, the ancient Roman religion, while reforming it in the hope of making it the driving force behind the empire.[2] Julian's religious beliefs were syncretic and he was an initiate of at least three mystery religions. But Julian's religious open-mindedness did not extend to Christianity due to its belief that it had an exclusive perspective on religious truth. Believing itself to be the "only true religion", Christianity was fundamentally incompatible with syncretic paganism.[3][4]
As emperor, Julian sought to turn the tide in the attempted suppression of non-Christian religions.[3][5] ... However, no Christian was allowed to teach or study the ancient classical authors, "Let them keep to Matthew and Luke", thus ending any chance they had of a professional career.[3][7] He did not believe Christians could honestly teach subjects replete with allusions to Greek deities whose existence they denied.[8] The Jewish historian and theologian Jacob Neusner writes: "It was only after the near catastrophe of Julian's reversion to paganism that the Christian emperors systematically legislated against paganism so as to destroy it."[9]
"In the eighteen brief months that he ruled between 361 and 363, Julian did not persecute [Christians], as a hostile tradition contends. But he did make clear that the partnership between Rome and Christian bishops ... was now at an end, replaced by a government that defined its interests and those of Christianity as antithetical."[10]
Source: Restoration of paganism from Julian until Valens - Wikiwand