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.

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.

Afterward, the Dominican said to the future pope: "My son, you one day will be called to be the ornament of your house and of your country.

Borgia cared strongly for the reestablishment of the unity of the church and his influence with the Aragonese monarch was the factor that allowed for the conclusion of the accord between the king and the new pope.

After his coronation, he rode a white horse through the streets of the city and followed the ancient custom, known as Monte Giordano, where representatives of the Jews met with the pope and presented him with the roll of the law.

[8] Not quite two years after the Fall of Constantinople in 1453, Callixtus was chiefly concerned with the organization of Christian Europe against an invasion by the Ottoman Empire.

At the time, it was said that Callixtus III "speaks and thinks of nothing but the crusade", spending hours discussing the topic with a warlike zeal.

Papal nuncios were dispatched to all the countries of Europe to beseech the princes to join once more in an effort to check the danger of a Turkish invasion.

Missionaries were sent to England, France, Germany, Hungary, Portugal, and Aragon to preach the Crusade, and to engage the prayers of the faithful for the success of the enterprise.

It was by order of Callixtus III that the bells were rung at midday to remind the faithful that they should pray for the welfare of the crusaders.

In 1456 the pope issued the papal bull Inter Caetera (not to be confused with Inter Caetera of 1493), reaffirming the earlier bulls Dum Diversas and Romanus Pontifex which recognized Portugal's trade rights in territories it had discovered along the West African coast.

[11] Callixtus III canonized the following four saints during his pontificate: Vincent Ferrer (3 June 1455), Osmund (1 January 1457), Albert of Trapani (15 October 1457), and Rose of Viterbo (1457).

According to one story that first appeared in a 1475 posthumous biography and was subsequently embellished and popularized by Pierre-Simon Laplace, Callixtus III excommunicated the 1456 appearance of Halley's Comet, believing it to be an ill omen for the Christian defenders of Belgrade from the besieging armies of the Ottoman Empire.

The 29 June 1456 papal bull of Callixtus III calling for a public prayer for the success of the crusade makes no mention of the comet.

Catholic historian Ludwig von Pastor opined: "Except for his nepotism, Calixtus III deserves high praise, more especially for the energy, constancy and purpose which he displayed in dealing with the burning question of the day – the protection of Western civilization from the Turkish power.

In this matter he gave a grand example to Christendom, and it is to be observed that in the midst of the military and political interest which claimed so large a share of his time and attention, he did not neglect the internal affairs of the Church, and vigorously opposed heresies.