John Stackhouse says the Roman Empire saw the early church as a threat.
Christians were persecuted in the Roman Empire partly because they were badly understood, and rumours would circulate – one of the more astounding rumours is that they would sacrifice infants and drink their blood and eat them, in ritual cannibalism. And this you can sort of see semi-humorously as a conflation of infant baptism and the Lord’s Supper, of taking the body and blood of Jesus through the bread and wine of Christian Eucharist. So part of the problem was sheer ignorance, they just didn’t know what these Christians were up to.
But the main reason they were persecuted is that they failed to conform to what Rome really insisted everyone conform to: namely, supreme loyalty to the Caesar and to Rome itself. You could do pretty much anything you liked in your private religious life – and Romans did – and the Roman government didn’t care. But they really cared that you got in line behind Caesar. And when Christians say Jesus is Lord, instead of Caesar is Lord, they’re seen as seditious. They’re literally breaking off from the state, and therefore a threat to common order.