Formed and educated in the family home, where they did not lack riches or moral training, he gave proof of a spirit very open to study and to piety.

After having renounced, by means of a notarized document, the principality which belonged to him by birth, in favour of his younger brother, as well as his very large inheritance, Tomasi was admitted into the Theatines, a religious Order founded by St. Cajetan of Tiene in 1524, as a reform movement in the Catholic Church, and noted for the simplicity of life followed by its members.

To a wide knowledge of Greek, he united the study of Ethiopic, Arabic, Syriac, Aramaic and Hebrew—converting his teacher, a Jewish rabbi, to Christianity.

The same pope created him Cardinal Priest, with the title of the Church of Santi Silvestro e Martino ai Monti, commanding him to accept the honour.

Silvestro e Martini ai Monti, are presently exposed for the veneration of the faithful in the Basilica of San Andrea della Valle of the Theatine Fathers, in Rome.

In truth, not a few of the norms, established by the authority of the Roman Pontiffs and by the documents of the Second Vatican Council and today in use in the Church, were already proposed and ardently desired by Tomasi, among which it is sufficient to recall: All these were intended to promote a more intimate and personal participation of the people in the celebration of the liturgy.

Chief among his publications are the Codices sacramentorum nongentis annis antiquiores (Rome, 1680), partly transcribed by Mabillon in his Liturgia Gallicana.

Following these, in order of time, were: Psalterium (Rouse, 1683), according to the Roman and Gallican editions, published under the name of Giuseppe Maria Caro.