Saint Cajetan was son of Gaspar, lord of Thienna, and Mary Porta, persons of the first rank among the nobility of the territory of Vicenza, born October 1480 at Vicenza. He took his degree as doctor utriusque juris at Padua in his twenty-fourth year. In 1506 he became at Rome a prothonotary Apostolic in the court of Julius II. Being much delighted with the end proposed by the confraternity in Rome, called of the love of God, which was an association of zealous and devout persons who devoted themselves by certain pious exercises and regulations to labour with all their power to promote the divine honour, he enrolled himself in it.

Though remarkable for his intense love of God, he did not advance to the priesthood till 1516. Recalled to Vicenza in the following year by the death of his mother, he founded there a hospital for incurables, thus giving proof of the active charity that filled his whole life. He also founded the Hospital of the Incurables of Venice, together with the Venetian patricians Maria Malpier and Marina Grimani, in 1522, and also attending that of S. Giacomo di Augusta, in Rome.

But his zeal was more deeply moved by the spiritual diseases that, in those days of political disorder, infected the clergy of all ranks, he strove to reform them by instituting a body of regular clergy, who should combine the spirit of monasticism with the exercises of the active ministry.Returning to Rome in 1523 he laid the foundations of his new congregation, which was canonically erected by Clement VII in 1524. One of his four companions was Gian Pietro Caraffa, Bishop of Chieti (in Latin Theate), afterwards Paul IV, who was elected first superior, and from whose title arose the name Theatines. The order grew but slowly..