"We are horrified to see what monstrous doctrines and prodigious errors are disseminated far and wide in countless books, pamphlets, and other writings which, though small in weight, are very great in malice. ...'We must fight valiantly,' Clement XIII says in an encyclical letter about the banning of bad books, 'as much as the matter itself demands and must exterminate the deadly poison of so many [bad] books; for never will the material for error be withdrawn, unless the criminal sources of depravity perish in flames."
(Mirari Vos, Encyclical of Pope Gregory XVI, 1832 A.D.)