Pope Pius IX, born Giovanni Maria Mastai Ferretti in 1792, served as the head of the Roman Catholic Church from 1846 to 1878. He is known for being the longest-reigning pope in history, leading the Church through significant changes, including the rise of nationalism in Europe and the loss of the Papal States. During his papacy, Pius IX convened the First Vatican Council in 1869, which defined the doctrine of papal infallibility. He also proclaimed the dogma of the Immaculate Conception in 1854, affirming that Mary, the mother of Jesus, was conceived without original sin.