Why Did Rome Start Persecuting Christians?
Rome was known for many things but religious persecution was not one of them. While Rome had a state religion and an imperial cult it did not generally outlaw foreign religion or religious practice. Doing so would have been disastrous. At its peak the Roman Empire stretched from England to Persia, Gaul to North Africa. The Roman Empire had millions of citizens that held religious beliefs of all types. Persecuting religious minorities would have led to the persecution of a large section of Roman society.
Instead, Rome simply asked that you paid homage to the emperor and acknowledged the Roman gods before or during any of your personal religious ceremonies. A vast majority of the world was polytheistic at this point and throwing out some platitudes to another pantheon of gods was not that big of an issue. In fact, the only group of people who would have taken umbrage with the prevalence of polytheism were the Jews who got special religious exemptions from Rome for just that reason.
If Rome was so tolerant to foreign religions why did the empire start cracking down on Christians? Why did Rome start persecuting the new religion when they had not before?
The Jewish Exemption
The Jews were the only large group of monotheistic worshipers in the Roman empire. Unlike the myriad of pagan tribes that populated Europe at the time the Jews believed in one single God. The God. Their vehement belief in monotheism forced Rome to give them special permission to worship differently than other pagans. Rome was fine with this because Jews do not look to convert others to their religion. They never had and they still don’t today.
In the early days of Christianity the Roman Empire regarded Christians as an official offshoot of Judaism. That meant that they got the same special treatment and protections as the Jews did.
However, after the Siege of Jerusalem when the Christians left the city ahead of the advancing Roman army the Jews in the empire made it very clear that Christians were not Jews. Not only were they not Jewish, they were currently on the outs with Judaism and should not even be regarded as kin.