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Why was the First Vatican Council suspended before completing most of its planned work?
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papal infallibility
In 1869 Pope Pius XI (center) convened an assembly of bishops that became known as the First Vatican Council (or Vatican I). The council declared the dogmas of papal primacy and papal infallibility before the assembly was suspended indefinitely in 1870.
The First Vatican Council was suspended indefinitely on October 20, 1870, by Pope Pius IX because Piedmontese troops occupied Rome. The council had only completed a small portion of its planned work, having promulgated two doctrinal constitutions: Dei Filius and Pastor Aeternus.1
The Pastor Aeternus decree formally defined the doctrine of papal infallibility, stating that the pope possesses supreme teaching power and is preserved from error when definitively teaching a doctrine concerning faith or morals for the whole church. This definition followed significant debate, with a majority of council fathers approving it despite opposition arguing that such a formal definition was inopportune.12