Joseph Mary Tomasi was born at Licata, in Sicily, the Diocese of Agrigento, on 12 September 1649, the first son of Julius Tomasi and Rosalie Traina Drago, the Prince of Lampedusa and the Duke of Palma di Montechiaro, and the baroness of Torretta e Falconeri. His own life was oriented toward God from his first years
But his own spirit aspired, even from youth, to be small in the Kingdom of God, and to serve not the kings of the earth but the King of heaven. He cultivated his pious desire in his heart until he obtained the consent of his father to follow his vocation to the religious life.
After having renounced, by means of a notarial document, the principate, which belonged to him through heredity, and his very rich patrimony, he was admitted into the Order of the Clerics Regular Theatine, founded by St. Cajetan of Thien in 1524. He made his religious profession in the Theatine house of St. Joseph, at Palermo, on 25 March 1666.
In the new state of life, which he had embraced to follow the call of Christ, he was able to dedicate himself better to piety and study. The sacred Liturgy had been his attraction from childhood.
He completed his studies of philosophy in Messina, Ferrara, Bologna and Modena, forced to the transfers for reasons of health. He studied Theology instead at Rome, in the House of San Andrea della Valle. In Rome, after having received the subdiaconate and the diaconate, on the Saturday of Advent, on 23 December 1673, he was ordained a Priest in the Lateran Basilica, at the hands of Mons. Joachim De Angelis, Archbishop of Urbino, Vice-Regent of the Cardinal Vicar Gaspar Carpegna. Two days later, on the night of the Nativity, he celebrated his first Mass, in the church of San Silvestro al Quirinale, at that time the residence of the General House of the Theatine Fathers.