Reading about the remarkable reign of the Emperor Diocletian prompts some speculation regarding the causes of the persecutions of the Christians during his principate and at other times during the Roman Empire.
The empire was normally fairly tolerant when it came to religious worship. However, it disliked certain forms of worship. Prior to the advent of Christianity, Rome banned some cults it considered weird and outlandish and so, of course, un-Roman. For example, the thought of being, literally, washed in the blood of the bull (so to speak) was initially frowned on, and the tendency of certain male worshipers of the Great Mother to castrate themselves created some not unnatural distress. Most of the banned religions managed to sneak back into the Roman world, however, and even came to be accepted in the sense that no effort was made by the emperors to stamp them out.
Christianity, though, was subject to persecution more than once. It was certainly considered weird, at one point. There were all sorts of stories regarding orgies and cannibalism being part of Christian worship, also incest (orgies themselves being, perhaps, not necessarily objectionable to the Roman mind). But, there had to be more involved for certain of the emperors to react as they did.
And there was, of course. Christians, or at least some of them, refused to pay homage to the emperor in the religious sense, generally by sacrifice. It's thought that this in particular made them the subject of persecution. This makes some sense, as Christians therefore would be considered to be against, if not a threat to, the emperor and thus the empire.
What I find interesting is why Christians would refuse to engage in this act, which probably required nothing in the nature of sincere belief or intent, and was simply a ritual gesture. The answer that they refused because they were good, or wise, or honest, and the empire and/or emperor bad, and the emperor not divine, is generally given. But I think there was more involved.
The Christians worshiped a God who became human. The possibility that a human could be divine, therefore, would presumably not be objectionable to them. However, their religion provided that only a particular human could be divine, and no other. In this sense, the Christian view of a divine human differed from the Greek and Roman view which had been prevalent for some time. The Olympian gods obviously had various human offspring. The emperors had been granted divine status on their death for some time (some, like Caligula, claiming it during their lifetimes). The divine status of the emperor was not objectionable to most pagans; they were not unused to the thought of humans having or asserting divinity in some sense. Christians, however, at least those who were not Arians, believed that the one God had become human only once, in the form of Jesus. And, as the Arians cam to learn, the orthodox Christians strongly objected to the idea of Jesus having a limited divinity--he was the same in substance with the Father, not merely similar. The thought of someone being divine but not God was foreign to Christianity.
When Diocletian as part of his efforts to reform the empire sought to protect the emperor from assassination and deposition by formally taking on divine status, he struck at the heart of the Christian doctrine. If only one human could be divine, the emperor could not be. In this and in other things Christianity was (is) an exclusive religion. There was only one God, all others therefore were false, and evil. There could be only one human who was divine, and all other claimants therefore were false, and evil. The idea of someone who was not Jesus claiming to be divine was inherently inconsistent with Christian doctrine. It does not justify the persecutions, but it seems to be true that Christianity's own intolerance of other gods, and other religions, played a role in its persecution under the empire. Certainly it explains its persecution of pagans, and Jews, and heretical Christians, after it subsumed the empire itself