He entered the order of the Dominicans at the age of sixteen, and ten years later became doctor of theology at Padua, where he was subsequently professor of metaphysics. A public disputation at Ferrara (1494) with Pico della Mirandola gave him a great reputation as a theologian, and in 1508 he became general of his order. For his zeal in defending the papal pretensions against the council of Pisa, in a series of works which were condemned by the Sorbonne and publicly burnt by order of King Louis XII, he obtained the bishopric of Gaeta, and in 1517 Pope Leo X made him a cardinal and archbishop of Palermo.
The year following he went as legate into Germany, to quiet the commotions raised by Luther. It was before him that the Reformer appeared at the diet of Augsburg; and it was he who, in 1519, helped in drawing up the bull of excommunication against Luther. Cajetan. was employed in several other negotiations and transactions, being as able in business as in letters. In conjunction with Cardinal Giulio de' Medici in the conclave of 1521-1522, he secured the election bf Adrian Dedel, bishop of Tortosa, as Adrian VI.
Though as a theologian Cajetan was a scholastic of the older Thomist type, his general position was that of the moderate reformers of the school to which Reginald Pole, archbishop of Canterbury, also belonged; i.e. he desired to retain the best elements of the humanist revival in harmony with Catholic orthodoxy illumined by a revived appreciation of the Augustinian doctrine of justification. Nominated by Clement VII a member of the committee of cardinals appointed to report on the "Nuremberg Recess," he recommended, in opposition to the majority, certain concessions to the Lutherans, notably the marriage of the clergy as in the Greek Church, and communion in both kinds according to the decision of the council of Basel.
In this spirit he wrote commentaries upon portions of Aristotle, and upon the Summa of Aquinas, and towards the end of his, life made a careful translation of the Old and New Testaments, excepting Solomon's Song, the Prophets and the Revelation of St John.
In contrast to the majority of Italian cardinals of his day, Cajetan was a man of austere piety and fervent zeal; and if, from the standpoint of the Dominican idea of the supreme necessity of maintaining ecclesiastical discipline, he defended the extremist claims of the papacy, he also proclaimed that the pope should be "the mirror of God on earth."
See "Aktenstucke uber das Verhalten der romischen Kurie zur Reformation, 1524-1531," in Quellen und Forschungen (Kön. Press. Hist. Inst., Rome), vol. iii. p. 1-20; TM Lindsay, History of the Reformation, vol. i. (Edinburgh, 1906).