“This shameful font of indifferentism gives rise to that absurd and erroneous proposition which claims that liberty of conscience must be maintained for everyone. It spreads ruin in sacred and civil affairs, though some repeat over and over again with the greatest impudence that some advantage accrues to religion from it. ‘But the death of the soul is worse than freedom of error,’ as Augustine was wont to say.[ St. Augustine, epistle 166.] When all restraints are removed by which men are kept on the narrow path of truth, their nature, which is already inclined to evil, propels them to ruin. Then truly ‘the bottomless pit’[ Ap 9.3.] is open from which John saw smoke ascending which obscured the sun, and out of which locusts flew forth to devastate the earth. Thence comes transformation of minds, corruption of youths, contempt of sacred things and holy laws — in other words, a pestilence more deadly to the state than any other. Experience shows, even from earliest times, that cities renowned for wealth, dominion, and glory perished as a result of this single evil, namely immoderate freedom of opinion, license of free speech, and desire for novelty.
“Here We must include that harmful and never sufficiently denounced freedom to publish any writings whatever and disseminate them to the people, which some dare to demand and promote with so great a clamor. 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 are in tears at the abuse which proceeds from them over the face of the earth. Some are so carried away that they contentiously assert that the flock of errors arising from them is sufficiently compensated by the publication of some book which defends religion and truth. Every law condemns deliberately doing evil simply because there is some hope that good may result. Is there any sane man who would say poison ought to be distributed, sold publicly, stored, and even drunk because some antidote is available and those who use it may be snatched from death again and again?
“The Church has always taken action to destroy the plague of bad books. This was true even in apostolic times for we read that the apostles themselves burned a large number of books.[ Acts 19.] It may be enough to consult the laws of the fifth Council of the Lateran on this matter and the Constitution which Leo X published afterwards lest ‘that which has been discovered advantageous for the increase of the faith and the spread of useful arts be converted to the contrary use and work harm for the salvation of the faithful.’[Acts of the Lateran Council 5, session 10, where the constitution of Leo X is mentioned; the earlier constitution of Alexander VI, Inter multiplices, ought to be read, in which there are many things on this point.] This also was of great concern to the fathers of Trent, who applied a remedy against this great evil by publishing that wholesome decree concerning the Index of books which contain false doctrine.[Council of Trent, sessions 18 and 25.] ‘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 books; for never will the material for error be withdrawn, unless the criminal sources of depravity perish in flames.’[Letter of Pope Clement XIII, Christianae, 25 November 1766.] Thus it is evident that this Holy See has always striven, throughout the ages, to condemn and to remove suspect and harmful books. The teaching of those who reject the censure of books as too heavy and onerous a burden causes immense harm to the Catholic people and to this See. They are even so depraved as to affirm that it is contrary to the principles of law, and they deny the Church the right to decree and to maintain it.”
-Pope Gregory XVI, Mirari Vos, 15 August 1832