I've heard that Julian the Apostate was the last pagan emperor of Rome, and after him there was no serious resistance to the spread of Christianity. Is this true? After he died, is there no evidence that pagans fought back against the growing power of Christianity?
I'm just wondering if there were maybe any pagan rebellions/riots/etc or short-lived attempts by pagan governors to restore the old religion, before Roman paganism was erased forever.
There was resistance throughout late antiquity all around the empire, at least if we believe often sensationalist Christian sources. Saints' lives often have the saints confronting pagans for example; and the Gothic Wars (6th century), when Rome was under siege a group of people secretly opened the gates of the temple of Janus, which was considered an important ritual to take before war. When Rome was sacked in 410 by the Vandals, the backlash against Christianity was so bad that Augustus wrote City of God to show Romans that, contrary to the arguments of the pagans, the Roman Empire was no worse for shirking paganism but had been pretty bad all along.
One of the most significant is the fight over the Altar of Victory that took place in 382. The emperor Gratian removed the Altar of Victory from the Senate house where it was used by the Senators to swear oaths upon and perform minor rituals. It had been a target in the past but Julian protected it from Christian activists.
When it was removed in 382, a group of senators led by Symmachus petitioned the emperor to restore it and Ambrose the bishop of Milan wrote a response. Amazingly, both sides of the exchange (although not the entire exchange) survive! This provides great insight into the mind of pagans who were afraid to give up their culture.