Persecution

An ancient Roman joke:

'If the River Tiber reaches the walls, if the River Nile does not rise to the fields, if the sky does not move or the earth does, if there is famine, if there is plague, the cry is at once: "The Christians to the lion!"
What, all of them to one lion?'
Tertullian, Apology.

Among the many temples to different Gods in a Roman town there would be one set aside for the Emperor. Everyone was expected to show their loyalty to the empire by standing before the statue of the Roman Emperor, putting a pinch of incense on the altar fire and saying "Caesar is Lord". They would be given certificates (libelli) to prove they had done it.

The only people who didn't have to do this were the Jews. At first the Romans thought Christians were the same as Jews, but as more and more non-Jews (Gentiles) became Christians, the Roman authorities started to persecute Christians.