Why were christians persecuted in ancient rome?

While there were some time periods that the Christians enjoyed relative peace after the death of Christ, for much of the first few centuries AD, Christians were persecuted and even killed. 
However, in the minds of the Romans, this was justified. For thousands of years before Christianity, the worship of multiple gods and goddesses was considered the norm, and the explanation for why the world was the way it was. 
Many Roman emperors ordered the extermination of Christians because they believed it went against the worship of their religion, and what was considered normal at the time. 

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