Tommaso Caccini, nicknamed Cosimo, was born in Florence in 1574 and entered the Dominican Order of Saint Mark at only fifteen years of age. He early on displayed great talents as a preacher and from the time of his novitiate delivered the Lenten sermons. The first attacks against Galileo had been launched by his fellow friar Niccolò Lorini (1544-1617) in 1612, but without any serious consequences. On the fourth Sunday of Advent of 1614, Tommaso Caccini denounced mathematics publicly as a devil's craft and a font of heresy, concluding his sermon with an unmistakable pun based on a quotation from chapter one of the Acts of the Apostles: «Viri Galilei, quid statis adspicientes in caelum?». Caccini was probably instigated by another opponent of Galileo's, Lodovico delle Colombe (1565-?). In the same period Niccolò Lorini had sent to Cardinal Paolo Camillo Sfrondati of the Congregation of the Index a copy of the Letter to Benedetto Castelli, in which Galileo dealt with the theme of the relationship between science and faith. On March 20, 1615 Tommaso Caccini reported voluntarily to the Holy Office to testify against Galileo. He first stated his opinion that the Copernican system was heretical and, second, that he had knowledge that Galileo's circle professed teachings that went against the faith. Without being interrogated on the subject, he also testified against the members of the Lyncean Academy, whom he accused of corresponding with German scientists and therefore of harboring Lutheran sympathies. To confirm these suspicions two prelates were called to testify, Ferdinando Ximenes and Giannozzo Attavanti. While the former supported Caccini's statements, the latter denied the basis of all the accusations. When the Holy Office chose not to proceed against Galileo, Tommaso Caccini, realizing that all his scheming had been for nought, decided to wait for Galileo, who was coming to Rome to better clear himself of suspicion. He took the occasion to offer Galileo his flimsy excuses and to promise to make it up to him in the future.

On his return to Florence Caccini became confessor to the nuns of the Convent of Orsina and Penitentiary at Santa Maria Maggiore. In 1622 he was licensed to teach theology and chosen as Prior of San Marco. He died in 1648 and was buried in the church of Santa Croce.