He began his ministry in the diocese of Albarracín, where Bishop Gaspar Juan de la Figuera appointed him his theologian, confessor, synodal examiner, and procurator.

[1] In October 1585, Bishop de la Figuera was sent as apostolic visitor to the Abbey of Montserrat and Calasanz accompanied him as his secretary.

In Rome, Calasanz found a protector in Cardinal Marcoantonio Colonna, who chose him as his theologian and, once he had learned to express himself in Italian, entrusted him with the spiritual direction of his household.

[3] The city of Rome offered many opportunities for works of charity, especially for the instruction of neglected and homeless children, many of whom had lost their parents.

Pope Clement VIII began making an annual contribution and many others gave their financial support to the work so that in a short time Calasanz had about 1,000 children under his charge.

In 1602, he rented a house near Sant'Andrea della Valle, commenced a community life with his assistants, and laid the foundation of the Order of the Pious Schools or Piarists.

One year later, on March 6, 1617, Pope Paul V approved Calasanz's group as the Pauline Congregation of the Poor of the Mother of God of the Pious Schools,[7] the first religious institute dedicated essentially to teaching, by his brief "Ad ea per quae."

The habits were paid for by the Cardinal Protector Justiniani, who with his own hands invested Joseph Calasanz in the chapel of his palace.

Emphasizing love, not fear, St. Joseph wrote: "if from the very earliest years, a child is instructed in both religion and letters, it can be reasonably hoped that his life will be happy."

While residing in Rome, Joseph endeavored to visit the seven principal churches of that city almost every evening, and also to honor the tombs of the Roman martyrs.

During one of the city's many outbreaks of plague, a holy rivalry existed between him and St. Camillus in aiding the sick and in personally carrying away for burial the bodies of those who had been stricken.

On account of his heroic patience and fortitude in the midst of trouble and persecution, he was called a marvel of Christian courage, a second Job.

After convincing the pope of the need to approve a religious order with solemn vows dedicated exclusively to the education of youth, the congregation was raised to that status on November 18, 1621, by a papal brief of Pope Gregory XV, under the name of Ordo Clericorum Regularium Pauperum Matris Dei Scholarum Piarum (Order of Poor Clerics Regular of the Mother of God of the Pious Schools).

In addition, according to the wishes of St. Joseph, members of the Order also profess a fourth vow to dedicate their lives to the education of youth.

In the Duchy of Lorraine, a similar project was being undertaken simultaneously by the Augustinians Peter Fourier and Alix Le Clerc, whose educational heritage was carried to New France.

Calasanz displayed the same moral courage, in his attitude to victims of the Inquisition, such as Galileo and Campanella, and in the acceptance of Jewish children in his schools, where they were treated with the same respect as other pupils.

So great and universal was Calasanz's prestige that he was even asked by the Ottoman Empire to set up schools there, a request which he could not, to his regret, fulfill, due to a lack of teachers.

In terms of discipline, and contrary to the prevailing philosophy of his own and subsequent eras, Calasanz favored the mildest punishment possible.

When Galileo fell into disgrace, Calasanz instructed members of his congregation to provide him with whatever assistance he needed and authorized the Piarists to continue studying mathematics and science with him.

Calasanz, with his courage and open-mindedness, invited the controversial thinker to Frascati to help teach philosophy to his teachers.

His pedagogical idea of educating every child, his schools for the poor, his support of the heliocentric sciences of Galilei and his service towards children and youth all aroused the opposition of many among the governing classes in society and the ecclesiastical hierarchy.

Problems were exacerbated, however, by Father Stefano Cherubini, originally headmaster of the Piarist school in Naples, who systematically sexually abused the pupils in his care.

However, he knew what Cherubini had really been up to, and he wrote that the sole aim of the plan was "to cover up this great shame in order that it does not come to the notice of our superiors".

Calasanz always remained faithful to the Church and died August 25, 1648,[7] at the age of 90, admired for his holiness and courage by his students, their families, his fellow Piarists, and the people of Rome.

Pope John Paul II affirmed that Saint Joseph Calasanz took as a model Christ, and he tried to transmit to youth, besides the profane sciences, the wisdom of the Gospel, teaching them to grasp the loving harmony of God.

Calasanz is also commemorated in a number of schools around the world, named after him and overseen by the Piarists and other religious institutes that have him as their patron saint.

Because August 25 falls during summer vacation in many schools, the Order in its official "Calendarium Ordinis" celebrates November 27 as the "Patrocinium" of St. Joseph Calasanz.