The First Ecumenical Council of the Vatican, commonly known as the First Vatican Council or Vatican I, was the 20th ecumenical council of the Catholic Church, held three centuries after the preceding Council of Trent which was adjourned in 1563. The council was convoked by Pope Pius IX on 29 June 1868, under the rising threat of the Kingdom of Italy encroaching on the Papal States. It opened on 8 December 1869 and was adjourned on 20 September 1870 after the Italian Capture of Rome. Its best-known decision is its definition of papal infallibility.

The Catholic Church, also known as the Roman Catholic Church, is the largest Christian church, with 1.39 billion baptized Catholics worldwide as of 2022. It is among the world's oldest and largest international institutions, and has played a prominent role in the history and development of Western civilization. The church consists of 24 sui iuris churches, including the Latin Church and 23 Eastern Catholic Churches, which comprise almost 3,500 dioceses and eparchies located around the world. The pope, who is the bishop of Rome, is the chief pastor of the church. The Diocese of Rome, known as the Holy See, is the central governing authority of the church. The administrative body of the Holy See, the Roman Curia, has its principal offices in Vatican City, a small independent city-state and enclave within the Italian capital city of Rome, of which the pope is head of state.