The Pope (the Bishop of Rome) is the successor of St. Peter, whom Jesus appointed as the head of the apostles with the words, 'You are Peter, and on this rock I will build my Church' (Matthew 16:18) and 'Feed my sheep' (John 21:17). Catholics believe the Pope exercises a unique role as the Vicar of Christ on earth, the visible head of the Church who ensures unity in faith and governance. The Pope has the charism of infallibility when he speaks ex cathedra (from the chair of Peter) on matters of faith and morals — a doctrine defined at the First Vatican Council in 1870. This does not mean the Pope is sinless or that his every opinion is infallible; it is a very specific and rarely exercised authority (used definitively only twice: the Immaculate Conception in 1854 and the Assumption in 1950). The Pope also convenes ecumenical councils, appoints bishops, issues encyclicals, and serves as a moral voice on the world stage. The papacy has endured for nearly 2,000 years with an unbroken line of over 260 popes.