Pope Callixtus III (31 December 1378 – 6 August 1458), also known as Alfonso de Borgia (Spanish: Alfons de Borja), was head of the Catholic Church and ruler of the Papal States from 8 April 1455 to his death in 1458.
He is the most recent pope to have taken the pontifical name of "Callixtus" upon his election.
A member of the powerful Borgia family, Callixtus III was the uncle of Pope Alexander VI, whom he appointed to the College of Cardinals.
Borgia spent his early career as a professor of law at the University of Lleida; he later served as a diplomat for the Kings of Aragon.
He became a tutor for King Alfonso V's illegitimate son Ferrante.
After arranging a reconciliation between Alfonso and Pope Martin, Borgia was made Bishop of Valencia.
In 1444, Pope Eugene IV named him a cardinal, and Borgia became a member of the Roman Curia.Borgia was elected pope in 1455.
During the Siege of Belgrade (1456), Callixtus initiated the custom that bells be rung at midday to remind the faithful to pray for the crusaders.
The tradition of the noon bell still exists in some congregations.
He was also responsible for the retrial of Joan of Arc that saw her vindicated.