I agree that some rulers of ancient Rome attacked christianity in part because they feared it threatened their political power (not the case for Diocletian as I'll explain later). But if you'll look throughout the history of religious persecution in western monotheistic societies, you'll notice that most of state sponsored persecution was motivated by the same thing--not some newly minted religious zeal sparked by monotheism, but good old fashioned political considerations.

However Roman society in general did persecute the Christians because of their differing religious beliefs. There are numerous accounts of pagan mobs composed of ordinary citizens attacking early Christians. The everyday pagans didn't form mobs because they were worried about the instability of the political regime, but because Christianity challenged their core beliefs--the very definition of religious intolerance. In addition much of the persecution by the Roman state was carried out because it was demanded by the people, not the rulers--motivated by religious intolerance.

>It absorbed many other cults

You'll also note that the Dionysian mystery cult was banned by the state for almost 200 years before they absorbed it, and other cults were banned before it.

> For the Romans, religion wasn't necessarily a fanatical pursuit

That is completely untrue, sure there were Romans who were only half hearted about religion just as there are less enthusiastic practitioners in a monotheistic religion. But if you think religion wasn't a fanatical pursuit for the Romans you need to read more on the initiation rituals of the mystery cults.

Additionally the particular sadism with which Christians were met speaks directly to the fanaticism of their persecutors. Diocletian (who institutionalize Christian persecution) developed a hatred for christians because pagan fortune tellers told him they could no longer divine the outcome of battles due to Christian influence, and he began his full-scale persecution when priests of the Oracle of Apollo told him they too could no longer divine the future because of the influence of Christians.

The most severe Christian persecution of the Roman Empire was directly caused by religious fanaticism.

> monotheistic religions are inherently more intolerant because of their rejection of anything other than their singular god.

Polytheism didn't make the Romans inherently more tolerant. They added gods to their pantheon, but only after those gods were thoroughly Romanized. In short they were only tolerant of religion that was very similar to what already existed allowing for slight variation along the way.

How is adding a new god, who has been modified to fit your pantheon, any more tolerant than early Christians who adopted certain customs and dates from pagan celebrations?

Person A believes in a single all encompassing God and will not allow to worship any other. However he will make some concessions by adding a new feast day on the day you traditional celebrate your god and will incorporate some elements of your festival into it.

Person B believes in 20 gods with different spheres of influence. He will let you worship any other god so long as you change it so that it is nearly indistinguishable from his existing pantheon, and thus no longer really the god you worship.

How is person B inherently more tolerant?