He studied Scripture and theology without a master.
Historians and theologians generally have regarded Catharinus as a brilliant eccentric. At sixteen he became Doctor of Civil and Canon Law (Juris Utriusque Doctor) in the academy of Siena. After visiting many academies in Italy and France he was appointed (1508) a professor at Siena, and had among his pupils Giovanni del Monte, afterwards Pope Julius III, and Sixtus of Siena, a converted Jew who esteemed his master, yet severely criticized some of his writings.
About 1513 he entered the Order of Saint Dominic in the convent of Saint Mark, at Florence.
This may account for his independence, and his defence of opinions which were singular, especially in regard to predestination, the certitude of possessing grace, the residence of bishops in their dioceses, and the intention required in the minister of a sacrament. In the third, public, session (4 February 1546), Catharinus, pronounced a notable discourse, later published.
Notwithstanding attacks upon his teaching he was appointed Bishop of Minori in 1546, and, in 1552, Archbishop of Conza, Province of Naples. Pope Julius III, successor of Paul III, called Politi to Rome, intending, says Jacques Échard, to elevate him to the cardinalate, but he died before reaching Rome.
Pallavicini and other authoritties declare that the Council of Trent did not condemn his singular opinions.
He died at Naples.
He defended the Immaculate Conception of the Blessed Virgin.
