Pius IX issues the Syllabus of Errors, 8 December 1864

Background. The Syllabus of Errors was an index document issued by the Holy See under Pope Pius IX on 8 December 1864, simultaneously with the encyclical Quanta cura. It collected 80 propositions considered errors or heresies, pairing headings with references to prior documents. The content served as a rebuttal to liberalism, modernism, moral relativism, secularization, and the political emancipation of Europe from Catholic monarchies.

Political Context. Quanta cura was prompted by the September Convention of 1864 between the emerging Kingdom of Italy and the Second French Empire of Napoleon III. Italians supporting the Risorgimento viewed the French troops as an occupying army, and in the Convention France agreed to withdraw its military garrison from Rome in anticipation of war with Prussia. Separately, in August 1863 Count Charles Montalembert, a proponent of Liberal Catholicism, gave speeches in Mechelen, Belgium, presenting his view of the future of modern society and the Church; at the end of March 1864 he received a letter from Cardinal Giacomo Antonelli, Secretary of State, criticizing the Mechelen speeches.

Condemnations. The Syllabus of Errors was a strong condemnation of liberalism, modernism, moral relativism, secularization, separation of church and state, and other Enlightenment ideas. In Quanta cura, Pius IX listed condemned errors in an attachment called the Syllabus of Errors, which condemned secularism and religious indifferentism. The documents referenced by the Syllabus were also intended to rebut the political emancipation of Europe from the tradition of Catholic monarchies.