FYI, bboop, Constantine didn't legalize Christianity, he just made it the official religion of the Roman Empire. Christianity was persecuted, then tolerated, then persecuted, back and forth up until that time, by the Roman authorities. Most of the time, they were left alone as long as they didn't preach against the Emperor, like St. Sebastian.
Also, Julian the Apostate (ruled just 19 months 331-332 AD) tried to re-introduce the gods of Roman and Greek mythology, but had a bad accident (likely shot by one of his own soldiers) during a battle in Persia. Sort of like what happened to Caligula, who also outraged the Roman aristocracy.
I liked your post, but I have to nit-pick something with you. The Barbarians borrowed Roman Law and Roman culture. The Barbarians also supported the church, which was populated with descendants of Roman aristocrats that knew how to run Manors that fed the population. The Barbarians, however, did not really borrow the Roman political system. They did sort of adopt the king system, but their political and social structure was feudalism, which naturally developed from the tribal system for self defense reasons.