Since early childhood John had a very good education and at 18 went to Perugia to study Law. While he was away during a local feud his brothers were killed and the houses of his father and mother destroyed. When John finished his studies, King Ladislaw of Durazzo appointed him as counsellor to the Royal Court of Justice in Naples. He was sent as an ambassador to the Malatesta, who imprisoned him in the castle of Brufa. He tried to escape, was caught and thrown into a cell, secured by a chain to the wall and with his feet in the water.
After three days in prison, St Francis appeared to him and arose his religious vocation. After paying a ransom he was set free and went back to Capestrano to dissolve a promise of marriage he had made to a Capestranese girl, renounced all his possessions and in 1415 entered the Monteripido monastery near Perugia to become a Franciscan friar.
In Fiesole St. Bernardine of Siena, who advocated a reform of the Church in the name of Jesus, became his spiritual guide. "Giantudesco" openly defended Bernardino when, because of his devotion to the Name of Jesus, the Sienese saint was accused of heresy. He, too, took as an emblem the monogram of Christ the King in his hard-fought battles against heretics and infidels. The Pope appointed him "Inquisitor of the Fraticelli", and sent him as his ambassador to Austria, Bavaria, Poland, where the Hussite scourge was growing. In the Holy Land promoted the union of the Armenian church with Rome.
He became a great preacher and travelled widely through Italy and Europe to build monasteries in honor of St. Francis. In L'Aquila he started the construction of a hospital and, after St Bernardine's death, promoted the building of a Church in honor of his spiritual guide, and began the construction of a convent in Capestrano in 1447.
In 1451 Pope Callisto III entrusted to him the mission to contact all the nations of Europe to raise funds and build an army to stop the Turkish invasion. He was at the head of the Crusaders near Belgrade on 22 July 1456 when the Turkish army was defeated. For eleven days and nights he never left the battlefield. But this was to be his last feat. Three months later, on October 23, he died in Villach (near present-day Ylohk, Croatia), handing over to his faithful the wooden Cross, with the emblem of Christ the King, which is still preserved in L'Aquila.
Before dying he asked that his books and personal possessions should be brought back to Capestrano, where they are still kept in a library that Countess Cabella da Celano built especially for him.