I've always been rather unconvinced by the arguments of Julian as a great Emperor who could have "saved" the Empire from Christianity and therefore decline. I think this is a bit of a hangover from the Gibbon era, to be honest. Julian was certainly an intelligent man, but he absolutely lacked the common touch (witness the chaos of what happened in Antioch when he arrived), and was at best a decent millitary commander, certainly not another Julius Caesar or Belisarius. His mission into Persia did not experience anywhere near the level of success experienced by those of Trajan or Severus, and by time of Julian's death, the Romans were limping home after an embarrassing series of setbacks and defeats.
If he survives the Persian campaign, I really can't see him surviving long, or the Christian faith being particuarly severely challenged. Julian did not seek to return to outright persecution of the Christians in OTL, but I think ITTL, frustration at Christianity's refusal to die may push him down this path by about 370 or so. Persecution may then, paradoxically enough, push the Christians back together into a single community, rather than a number of squabbling ones, and thus strengthen the faith.