On 14 June 1966, Pope Paul VI abolished the Index of Prohibited Books which had first been established by Pope Paul IV in 1559.
Paul IV established the Index Liborum Prohibitorum as part of the Catholic Counter-Reformation. Fearing that certain books might lead to the corruption of the faith and morals of the Catholic faithful, Paul IV wanted the Index to prohibit those writings deemed to contain errors of theology or morality. Works that would be listed on the Index included scientific works such as texts by Johannes Kepler and Galileo Galilei; philosophic works by Jean Paul Sartre and René Descartes; and literary works by Victor Hugo and John Milton. The writings of Saint Faustina were placed on the Index for twenty years, though the Vatican later determined that the writings were placed on the Index due to a faulty Italian translation of St. Faustina’s words.
Some works that one might think would be on the Index were not. For example, books that contained teaching explicitly contrary to Catholic doctrine were automatically forbidden for Catholics to read and some works did not end up on the list simply because no one denounced them. The works that were placed on the Index, often did so after much debate. Catholic authors who ended up on the Index had the opportunity to defend their writings or to modify and then re-publish their works.
By the mid-20th Century, it was becoming impossible for the Congregation of the Doctrine of the Faith to read all the books that were being published and the Index was less and less able to fulfill its function. Therefore, the Congregation stated that the Index would no longer be issued. Catholics are still encouraged to avoid those writings which might threaten faith or morality.
Authors who ended up on the Index of Prohibited Books