By this exemplary course of life he incurred the hatred of his fellow pages; for his rigid rule of life was a check upon their profligacy. They ridiculed his piety, and heaped upon him persecution, not only by words, but also by blows. He bore their ill-treatment with great patience; but it produced an illness which brought him into hourly danger of death. At the very moment of his agony his patron, S. Francis Xavier, at the earnest prayer of his mother, restored his health. She had from his birth dedicated her son to the apostle of India, and she now vowed that if her child were to recover, he should wear the Jesuit habit for the whole year in honour of his deliverer. When, therefore, he appeared at court again after his recovery, it was in a little black robe, with a chaplet of the Blessed Virgin hanging by his side; and in this garb he served the Prince, and attended the Jesuit college of S. Antony of Padua. The people stopped in the streets to see him pass; not on account of the strangeness of such a dress on a child, but to mark his holy and edifying demeanour. When the term of his vow had expired, he put off the dress, but with the intention to assume it one day for ever. Notwithstanding his infirm health, he had long resolved to leave the world, and lead an apostolic life as a Jesuit, and at the age of fifteen he carried this resolution into effect. He applied for admission into the society to the father-provincial, Michael Tinsco, by whom he was placed in the novitiate at Lisbon. The Prince, Don Pedro, opposed his resolution, but the mother rejoiced that her son should possess a vocation for the Company of Jesus.
It was on the 17th of December, 1662, that John de Britto entered the novitiate at Lisbon. A novena had just commenced in preparation for Christmas. At its close each postulant had to present the Infant Jesus with a petition, according to his wants. De Britto wrote his petition with the others. It was that he might be sent as a missionary to Japan, there to live and labour, and at length obtain the crown of martyrdom. After two years he took the three vows of poverty, chastity, and obedience, having passed through the novitiate a very model of holiness.