The 1913 Catholic Encyclopedia describes Catharinus as a "brilliant eccentric", and asserts that, despite frequent accusations of heresy, his teachings were always "within the bounds of orthodoxy".
He had among his pupils Giovanni del Monte, afterward Pope Julius III, and Sixtus of Siena, a converted Jew who esteemed his master yet severely criticized some of his writings.
This may account for his independence and his defense of singular opinions, especially regarding predestination, the certitude of possessing grace, the residence of bishops in their dioceses, and the intention required in the minister of a sacrament.
He was a strenuous defender of Catholicism against Martin Luther and his followers and was prominent in the discussions of the Council of Trent, to which he was called by his former pupil, Cardinal del Monte, legate of Paul III.
In the third public session (4 February 1546), Catharinus pronounced a notable discourse, later published ["Oratio ad Patres Conc.