The efficacy of his prayers is strikingly shown in the conversion to the Catholic faith of a Lutheran prince, John Frederick, Duke of Brunswick. While visiting the principal courts of Europe, in the year 1649, the prince, then twenty-five years of age, came from Rome to Assisi expressly to see Joseph, of whose fame he had heard in Germany. On his arrival at the monastery, he (along with two of his retainers) was given lodging... and was led next morning to the door of the chapel, where the servant of God was saying Mass. The saint, who was not informed of their presence, was made aware of it when about to break the sacred host, which he found so hard, that, in spite of all his efforts, he could not break it, but had to replace it on the paten. Fixing his eyes upon the host, he wept and with a loud cry rose in kneeling posture about five paces into the air. With another cry he returned after some time to the altar and broke the sacred host, though with great effort. At the instance of the Duke, the Father Superior asked him why he had wept, and he replied: "My dear compatriot, the persons, whom you sent to my Mass this morning, have a hard heart; for they do not believe all that Holy Mother Church teaches, and therefore the Lamb of God was hardened in my hands so that I could not break the sacred host." The Duke, astonished at this occurrence, deferred his departure in order to consult with the servant of God. This he did after dinner, remaining with the saint till Compline... The following year he came to Assisi and, as he had promised, knelt before the Blessed Sacrament, and in the presence of Cardinals Facchinetti and Rapaccioli, made profession of faith in the hands of Father Joseph.

Saint Joseph of Cupertino
Fr. Angelo Pastrovicchi, O.F.M. Conv.