This doctrine was defined dogmatically at the First Ecumenical Council of the Vatican of 1869–1870 in the document Pastor aeternus, but had been defended before that, existing already in medieval theology and being the majority opinion at the time of the Counter-Reformation.
According to Catholic theology, there are several concepts important to the understanding of infallible, divine revelation: Sacred Scripture, Sacred Tradition, and the Sacred Magisterium (Teaching Authority). The infallible teachings of the Pope are part of the Sacred Magisterium, which also consists of ecumenical councils and the "ordinary and universal magisterium". In Catholic theology, papal infallibility is one of the channels of the infallibility of the Church. The infallible teachings of the Pope must be based on, or at least not contradict, Sacred Tradition and Sacred Scripture.
The doctrine of infallibility relies on one of the cornerstones of Catholic dogma: that of Petrine supremacy of the pope, and his authority as the ruling agent who decides what are accepted as formal beliefs in the Roman Catholic Church. The use of this power is referred to as speaking ex cathedra. The solemn declaration of papal infallibility by Vatican I took place on 18 July 1870. Since that time, the only example of an ex cathedra decree took place in 1950, when Pope Pius XII defined the Assumption of Mary as an article of faith. Prior to the solemn definition of 1870, there were other decrees which fit the definition of ex cathedra, for example, Pope Boniface VIII in the bull Unam Sanctam of 1302, and Pope Pius IX in the Papal constitution Ineffabilis Deus of 1854.
The church teaches that infallibility is a charism entrusted by Christ to the whole church, whereby the Pope, as "head of the college of bishops," enjoys papal infallibility. This charism is the supreme degree of participating in Christ's divine authority, which, in the New Covenant, so as to safeguard the faithful from defection and guarantee the profession of faith, ensures the faithful abide in the truth. The church further teaches that divine assistance is also given to the Pope when he exercises his ordinary Magisterium.
According to the teaching of the First Vatican Council and Catholic tradition, the conditions required for ex cathedra papal teaching are as follows: