Index Librorum Prohibitorum, list of books once forbidden byRoman Catholic Church authority as dangerous to the faith ormorals of Roman Catholics. Publication of the list ceased in 1966, and it wasrelegated to the status of a historical document.

Compiled by officialcensors, theIndex was an implementation of one part of the teaching function of the Roman Catholic Church: to prevent the contamination of the faith or the corruption ofmorals through the reading of theologicallyerroneous or immoral books. It was not, therefore, equivalent to the total legislation of the church regulating reading by Roman Catholics, nor was it ever a complete catalog of forbidden reading. Until 1966,canon law prescribed two main forms of control over literature: the censorship of books by Roman Catholics in advance of publication, in regard to matters of faith and morals (a practice still followed), and the condemnation of published books that were judged to be harmful. The works appearing on theIndex are only those thatecclesiastical authority was asked to act upon.

The origin of the church’s legislation concerning the censorship of books is unclear, but books were a source of concern as early as the scriptural account of the burning of superstitious books atEphesus by the new converts ofSt. Paul (Acts 19:19). The decree of PopeGelasius I about 496, which contained lists of recommended as well as banned books, has been described as the first Roman Index.

The firstIndex Librorum Prohibitorum was published in 1559 by the Sacred Congregation of the Roman Inquisition (aprecursor to the Congregation for the Doctrine of the Faith) in an attempt to combat the spread of some of the writings of the Protestant Reformation. The first printed Index included a prohibition against the “Bible in Castilian Romance or any other vulgar tongue,” a ban that remained in force until the 18th century. Many books deemed heretical or threatening to the faith were destroyed or hidden as a result of the Index and the accompanyinginquisitions, and hundreds of printers took flight to Switzerland and Germany.

In 1564 the church published the 10 “Tridentine Rules” to clarify its prohibitions on books not necessarily enumerated in theIndex, including against all heretical and superstitious writings, and to establish the punishment ofexcommunication for those in possession of such works. The Tridentine Rules wereabrogated by PopeLeo XIII in 1897 and replaced with new general decrees. TheIndex itself continued to have official sanction well into the 20th century. The last and 20th edition of theIndex appeared in 1948. The list was suppressed in June 1966, at which point it became a moral guide instead of obligatory law.

Perhaps the most dramatic form ofcensorship in Christendom, theIndex was not limited totheology: it banned works ranging from love stories to philosophicaltreatises topolitical theory. All the writings of certain authors—includingDavid Hume,Thomas Hobbes,Émile Zola, andJean-Paul Sartre—were prohibited, while only specific books by other authors were proscribed, such asJohn Milton’sParadise Lost andBlaise Pascal’sPensées. One or more works by nearly every modern Western philosopher were censored in theIndex, even those who professed a belief in God, such as Erasmus, René Descartes, Immanuel Kant, George Berkeley, andNicolas Malebranche. Other famous writers with banned books includedVoltaire,Edward Gibbon, Montesquieu,Giordano Bruno, Francis Bacon,Laurence Sterne, Daniel Defoe, Nicolaus Copernicus, and Níkos Kazantzákis. That the works of some atheist thinkers, notably Friedrich Nietzsche andArthur Schopenhauer, were not listed was because of the supplemental Tridentine ban on heretical works.