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 profes sor at Lerida; in 1429 he was made bishop of Valencia, and in a cardinal, owing his promotion mainly to his close friendship with Alphonso V., king of Aragon and Sicily. Chosen pope in April 1455, he endeavoured fruitlessly to organize a crusade. 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. Notorious for nepotism, he was responsible for introducing his nephew, Rodrigo Borgia, afterwards Pope Alex ander VI., to Rome. He died on Aug. 6, 1458.