The Secretary of State of His Holiness The Pope, commonly known as the Cardinal Secretary of State, presides over the Holy See Secretariat of State, which is the oldest and most important dicastery of the Roman Curia. The Secretariat of State performs all the political and diplomatic functions of the Holy See and the Vatican City. The Secretary of State is sometimes described as the prime minister of the Holy See, even though the nominal head of government of Vatican City is the President of the Pontifical Commission for Vatican City State.
The Cardinal Secretary is appointed by the Pope, and serves as one of his principal advisors. As one of the senior offices in the Roman Catholic Church, the Secretary is required to be a cardinal. If the office is vacant, a non-cardinal may serve as Pro-tem Secretary of State, exercising the powers of the Secretary of State until a suitable replacement is found or the Pro-Secretary is made a cardinal in a subsequent consistory.
The Cardinal Secretary's term ends when the Pope who appointed him dies or leaves office. During the sede vacante period, the former Secretary acts as a member of a commission with the Camerlengo of the Holy Roman Church and the former President of the Pontifical Commission for Vatican City State, which exercises some of the functions of the head of state of the Vatican City until a new Pope is elected. Once the new Pope is chosen, the former Secretary's role in the commission likewise expires, though he can be re-appointed as Secretary of State.
The office traces its origins to that of secretary intimus, created by Pope Leo X in the early 16th century to handle correspondence with the diplomatic missions of the Holy See, which were just beginning to become permanent postings instead of missions sent on particular occasions. At this stage the secretary was a fairly minor functionary, the Vatican administration being led by the Cardinal Nephew, the Pope's confidant usually taken from his family.
The imprudence of Pope Julius III in entrusting the office of Cardinal Nephew to his alleged lover Innocenzo Ciocchi Del Monte, a teenaged, virtually illiterate street urchin whom his brother had adopted a few years earlier, led to an upgrading of the Secretary's job, as the incumbent had to take over the duties the Cardinal Nephew was unfit for. By the time of Pope Innocent X the Secretary of State was always himself a Cardinal, and Pope Innocent XII abolished the office of Cardinal Nephew in 1692. From then onwards the Secretary of State has been the most important of the officials of the Holy See.
In 1968, Pope Paul VI's apostolic constitution Regimini Ecclesiae Universae further enhanced the powers of the Secretary, placing him over all the other departments of the Roman Curia. In 1973 Paul further broadened the Secretaryship by abolishing the ancient office of Chancellor of the Holy Roman Church and merging its functions into those of the Secretary.