Popery is an old, offensive English word for the Roman Catholic Church — the Christian church that has the Pope in Rome as its head. People who hated that church, especially Protestants in Britain hundreds of years ago, used it to make the whole faith sound like a bad system, not a real religion. It was a fighting word in real wars of religion, and some people still use it that way to hurt. You should never use it about people or their church. If you meet it in an old book, it tells you the writer is attacking Catholics, not naming the church in a calm way. The ordinary, respectful words are Catholic and Roman Catholic.

天主教（舊時蔑稱，極冒犯，不要用）