Urged by his particular love for the ancient documents of the Church and for the sound ecclesiastical traditions, he considered that a good part of his own religious perfection lay in dedicating himself, with the spirit of faith, to the publication of rare liturgical books and of the ancient texts of the sacred Liturgy, and so bringing to light many ancient sacred scriptures which until then had been hidden in the libraries. In fact, thanks to his wide knowledge of sacred matters, he edited many volumes dealing with biblical, patristic and principally liturgical subjects.

Besides being in relationship with important persons and scholars of his own intellectual breadth, he dedicated himself no less to the formation of the simple faithful. For these he composed a Breve istruzione del modo di assistere fruttuosamente al Santo sacrificio della Messa, as well as a condensed version of the Psalms selected and prepared for facilitating the prayer of the Christian.

Named General Consultor of his Order by his confreres, out of humility he quickly renounced the appointment. His many publications on liturgical subjects, in which piety was united with scholarship, motivated the titles which some of his contemporaries gave to him, such as that of “the Prince of the Roman Liturgists”

In truth, the intuitions of Father Tomasi anticipated not a few of the norms established by the Second Vatican Council (1962-1965) and today remarkably in use in the Church. All his labours and solicitudes, in research and in his studies, were not able in the slightest amount to distract Father Tomasi from aiming, constantly and with all his strength, at the attainment of that evangelical perfection to which God had called him from his infancy.

To all he was an example of profound humility, of the spirit of mortification and of sacrifice, of faithful observance, of meekness, poverty, piety, and filial devotion to the Blessed Virgin Mary. He helped the poor; he gave relief to the sick, both at home and in the hospital of St. John Lateran. In this way wisdom and charity were united and harmonized in him. Clement XI, named him cardinal, with the title of Ss. Silvestro e Martino ai Monti, in the Consistory of 18 May 1712. He accepted the cardinalate only through the expressed mandate of the Pope.

He joined to the cardinalatial dignity all those virtues which distinguished him as a Theatine religious; he changed none of his previous rule of life. For his court and for the service of his home he chose, for motives of humility, the poor, the weak, the lame and persons with various physical handicaps. Besides, he also dedicated himself to teaching the catechism of Christian doctrine to children and to the other faithful.He died a saintly death on 1 January 1713 in his apartment at the Passarini Palace on the Via Panisperna, Rome. Pius VII proclaimed him Blessed on 29 September 1803. John Paul II canonized him on 12 October 1986.

The relics of his body, transferred in 1971 from the Basilica of his title of Ss. 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. La sua festa liturgica è il 3 gennaio