CALIXTUS III  . (c . 1378-1458), pope from 1455 to 1458, was a Spaniard named Alphonso de Borgia, or Borja . A native of Xativa, he gained a great reputation as a jurist, becoming professor at Lerida; in 1429 he was made bishop of Valencia, and in 1444 a cardinal, owing his promotion mainly to his close friendship with Alphonso V., king of Aragon and Sicily . Chosen pope in April 1455, he was very anxious to organize a crusade against the Turks, and having sold many of his possessions, succeeded in equipping a fleet . Neither the princes nor the people of Europe, however, were enthusiastic in this cause, and very little result came from the pope's exertions . During his papacy Calixtus became involved in a quarrel with his former friend, Alphonso of Aragon, now also king of Naples, and after the king's death in June 1458 he refused to recognize his illegitimate son, Ferdinand, as king of Naples, asserting that this kingdom was a fief of the Holy See . This pope was notorious for nepotism, and was responsible for introducing his nephew, Rodrigo Borgia, afterwards Pope Alexander VI., to Rome . He died on the 6th of August 1458 . See A . Hauck's Realencyklopadie, Band iii . (Leipzig, 1897) .