Michael Grant, in The Fall of the Roman Empire (1990), writes:
Until the early fourth century AD, the official religion of the Roman world had been pagan. The ancient paganism of the Roman state was willing to be all things to all men. Being polytheistic, it was multiple and versatile. It was very far from exclusive. Nor was it generally intolerant. True, it had developed intolerance towards the Christians, because the Christians, sicne they owed loyalty to a Higher Master, seemed to be denying the sufficient minimum of loyalty to the Emperor and the nation. (p. 155)
This is the sort of statement that tends to be broadly proclaimed, and then the oppression of pagans and heretics by the orthodox, catholic Roman government is discussed in stark contrast to the happy go-lucky, tolerant polytheism of previous centuries.
It’s not, of course, entirely true.
It is certainly true that polytheisms have a certain tolerance. Speaking of ancient circumstances, their tolerance is for other polytheisms. They just make a bit of mental adjustment in terms of interpretatio and Ammon becomes Zeus Ammon. No big deal. Religious tolerance amongst polytheists was certainly real and certainly more visible than in the Christian empire of Late Antiquity. I wouldn’t want to make the case that the government under polytheists regulated belief and cult the way its Christian successors would.
However, polytheists in the ancient world were intolerant, first of all, to monotheists. The maltreatment of Christians and Jews in the Roman Empire, and of Jews under the Seleucids, tends to be painted largely in political terms these days. The ancient mind would reject that division between religion and politics. The Romans may have had a somewhat ambivalent relationship with official cultus, seen in Cicero, but this is precisely because religion and politics go hand in hand.
Thus, religious dissent in the ancient, Roman world is inevitably seen as political, even if we would say today that it’s not. To argue that the Roman persecution of Christians and Jews was wholly or primarily political is to miss how intertwined politics and religion were in the Roman mindset and the Roman world.
Jews and Christians are not, however, the only religious groups targeted by the Roman authorities. For example, in 186 BC the Senate passed a decree against the Bacchanalia — largely (we think) because it seemed to upset the traditional family loyalties, not necessarily because it had any overtly political aspect. The social outworkings of the Bacchic rites were seen as subversive and therefore this religious expression was suppressed.
Another group targeted was the ‘Gnostic’ religion of Manichaeism which Diocletian considered subversive and foreign (although the Roman world was well populated by foreign cults by AD 284). This is a religion that also suffered under the Christian empire.
These are the only two I can think of, although one cannot help but mention the execution of Socrates on what were essentially religious grounds.
Polytheism can certainly find more people with whom to agree than monotheism. But any time religion or ideology is so closely linked to the government that the government believes its own success is tied to that religion or ideology, then those of different ideologies suffer, whether Bacchanalians, Jews, Christians, and Manichaeans under Rome; Socrates in Athens; pagans, heretics, and Jews under Christian governments; Christians, Jews, and pagans under Islamic governments (add Buddhists under the Taliban and Yezidis under ISIS); or Christians, Buddhists, Jews, Muslims, Falun Gong, et al, under Communist governments.
Polytheism’s apparent tolerance is a most agreeable trait in today’s cultural climate. But let’s not forget that ideological dissenters in most governments in most of history, whether in ancient, pagan Rome or modern, Communist China, have suffered at the hands powerful government authorities.