Why were Christians persecuted in the Roman Empire? https://www.bbc.co.uk/history/ancient/romans/roman_religion_gallery_09.shtml The answer has to come from this article.
The Roman government's attitude to religion in the early centuries of the Common Era (C.E.) was a natural extension of its attitude to all forms of group activity. While most group activities were harmless or even beneficial to the state, some group activities led to civil unrest —consider the political protest groups of the modern era, whose purpose is to disrupt daily life in order to make themselves heard by their governments. The Roman government drew a very hard line against any attempts to subvert the state's authority. As the article points out,
Groups that met privately (whether religious sects, trade guilds or even local fire brigades!) were viewed with suspicion by Roman authorities, who suspected such groups of plotting subversion.
The Roman Empire was vast, and its citizens practiced many different religions. Rome tolerated most of these but reacted strongly against any religious groups that "threatened public order." The Jews of Roman-occupied Judea were a constant source of trouble to the Roman government. Christianity, which began as a Jewish sect, was therefore viewed suspiciously from its inception.
The particulars of the Christian faith made it even more suspect to the Roman authorities. Christians did not acknowledge other gods, and consequently would not observe state holidays or make sacrifices for the health of the emperor or the good of the empire. For many Roman citizens across the Empire, such sacrifices were probably performed as a public duty rather than a matter of personal spiritual belief. By refusing to perform their public duties in this regard, Christians were marking themselves out as citizens who did not care about the welfare of the state. This brought members of the new faith under serious scrutiny and often led to their persecution by the Roman authorities.
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