Famous as a preacher, theologian, and inquisitor, he earned himself the nickname "the Soldier Saint" when in 1456 at age 70 he led a Crusade against the invading Ottoman Empire at the siege of Belgrade with the Hungarian military commander John Hunyadi.

Elevated to sainthood, he is the patron saint of jurists and military chaplains, as well as the namesake of two Franciscan missions, one in Southern California and the other in San Antonio, Texas.

As was the custom of this time, John is denoted by the village of Capestrano, in the Diocese of Sulmona, in the Abruzzi region, Kingdom of Naples.

[1] He soon gave himself up to the most rigorous asceticism, vigorously defending the ideal of strict observance and insisting on purity of doctrine,[4] following the example set by Bernardine.

Unlike most Italian preachers of repentance in the 15th century, John was effective in northern and central Europe– in German states of Holy Roman Empire, Bohemia, Moravia, Austria, Hungary, Croatia, and the Kingdom of Poland.

This facet of his life is covered in great detail by his early biographers, Nicholas of Fara, Christopher of Varese and Girlamo of Udine.

He was frequently deployed to embassies by Popes Eugene IV and Nicholas V: in 1439, he was sent as legate to Milan and Burgundy, to oppose the claims of the Antipope Felix V; in 1446, he was on a mission to the King of France; in 1451 he went at the request of the emperor as Apostolic Nuncio to Austria.

As legate, or inquisitor, he persecuted the last Fraticelli of Ferrara, the Jesuati of Venice, the Crypto-Jews of Sicily, Moldavia and Poland, and, above all, the Hussites of Germany, Hungary, and Bohemia; his aim in the last case was to make talks impossible between the representatives of Rome and Bohemian "Hussite king" George of Podiebrad, for every attempt at conciliation seemed to him to be conniving at heresy.

[4] John, together with his teacher, Bernardine, his colleague, James of the Marche, and Albert Berdini of Sarteano, are considered the four great pillars of the Observant reform among the Friars Minor.

[10][11] However, “[t]hat in dealing with heretics and Jews he transgressed established bounds and thereby failed against Christian charity is a thought practically unknown to contemporaries.

Even Doering, one of his severest critics, finds nothing to blame in Capistrano’s behavior toward the Jews.”[12] After the fall of Constantinople in 1453, the Ottoman Empire, under Sultan Mehmed II, threatened Christian Europe.

That following year Pope Callixtus III sent John, who was already aged 70, to preach a Crusade against the invading Ottomans at the Imperial Diet of Frankfurt.

Although he survived the battle, John fell victim to the bubonic plague, which flourished in the unsanitary conditions prevailing among armies of the day.