First Vatican Council — Papal Infallibility

Convened by Pope Pius IX, the First Vatican Council was held against the backdrop of the rising secularism and rationalism of the 19th century. Its most significant achievement was the dogmatic constitution Pastor Aeternus, which formally defined the dogma of Papal Infallibility. This dogma states that when the Pope speaks ex cathedra (from the chair of Peter) to define a doctrine concerning faith or morals to be held by the whole Church, he is preserved from error by the Holy Spirit. The Council was abruptly suspended when Italian troops captured Rome, ending the Papal States.