Bart D. Ehrman, The Triumph of Christianity: How a Forbidden Religion Swept the World (Simon & Schuster: 2018), 179.
At the outset I need to stress that organized opposition to the Christians came, for the most part, in isolated incidents. The church did not experience perennial violent persecution. The idea that Christianity was an illegal religion under constant surveillance by the state apparatus that inflicted martyrdom on many thousands of believers, forcing the church underground into the Roman catacombs – all this is more the stuff of Hollywood than of history. Christianity was not declared illegal in the empire before the middle of the third century. There were no empire-wide laws or decrees issued by the central authorities in Rome that proscribed the faith. Christians did not, as a rule, go into hiding. For the most part, they lived perfectly normal lives in the midst of other religions. That catacombs were not meeting places for Christians forced to congregate in clandestine cells for fear of violent persecution. Persecution itself was rare, and there were relatively few casualties.