He says that they principal breeders are the Papists. This means they are the Catholics. He uses the word Papists because it is a demeaning term for people who follow the Pope. The Pope is the spiritual leader of the Catholic church. He calls them principal breeders because he believes all they do is have children.
2. criticize the growing influence of the catholic population
By saying good Protestants, he is inferring that the Catholics are not good. Instead of staying where they'd be forced to financially support "an episcopal curate", they leave.
