Stephenson’s does a superb job of explaining the impulse beyind the Diocletian persecution, which, he says, came from Diocletian and not, as Christians were inclined to say, from Galerius.
At the heart of the persecution was an effort to revive romanitas , understood in the narrow sense that Diocletian took the term. Diocletian “had only a poor, perhaps no, knowledge of Greek, the language spoken by citizens of his capital at Nicomedia and the lingua franca throughout the eastern provinces that he regularly toured. His Illyrian peasant upbringing and decades in the army camps made Latin his preferred tongue, and seeking to make a virtue of his shortcoming he made the revival of Latin a core element in his programme of cultural renewal. It had never been un-Roman to speak Greek, which was the core of a rounded education. But now all official business was to be conducted and recorded in Latin, even where all parties spoke Greek. The new emphasis on Latin was not to appear anti-intellectual: professors of Latin were appointed in the imperial capitals, and it is due largely to this revival that such a variety of Latin panegyrical material has survived, to elucidate the otherwise opaque decades of the Tetrarchy.”
Romanitas as a religious ideal for Diocletian:
“Living in the east and moving constantly through the provinces, Diocletian became increasingly intolerant of religious differences. He was no longer willing to countenance claims by Christians, like Origen, that they prayed in their own way for his well-being. Their forms of worship, their ways of being Roman, clashed with his own limited conception of romanitas .” He complained that “excessive leisure sometimes provokes ill-suited people to cross natural limits and encourages them to introduce false and outrageous forms of superstitious doctrine . . . . No new belief should criticize the religion of old.”
He oppose Manicheanism because he “found their religion to be too ‘Persian.’” And he eventually opposed the Christians on the grounds that, despite being Romans, they were not the sort of Romans that he desired.