After a thorough scientific and philosophical training, he began his career in the service of his native city. In 1521 he was the Republic's ambassador to Charles V. He accompanied Charles to Spain; later, after the sack of Rome, he assisted in reconciling the emperor and Clement VII, also the emperor and the Republic of Bologna. His accomplishments, but still more his mild resoluteness and blameless character, made him respected everywhere.
One of the fruits of his diplomatic activity is his De magistratibus et republica Venetorum. In 1535, Paul III unexpectedly made the secular diplomat a cardinal in order to bind an able man of evangelical disposition to the Roman interests. Contarini accepted, but in his new position did not exhibit his former independence. The disposition which Ranke (Popes, i. 118) calls "the collected product of all his higher faculties" governed his action also in the new field.
At first everything seemed to work well. In 1536 Paul III appointed a commission to devise ways for a reformation. The Protestant evangelical movement had made such progress in Italy that something had to be done, and it seemed best that the most influential be the agents. The decision was a bold one; Paul III, however, received favorably Contarini's Consilium de emendanda ecclesia, but it remained a dead letter, and his successor Paul IV, once a member on the commission, in 1539 put it on the Index, a deed which still embarrasses Catholic historians. What Contarini had to do with it is shown by his letters to the pope in which he complained of the schism in the church, of simony and flattery in the papal court, but above all of papal tyranny. But he came a century too late.
Contarini in a letter to his friend Cardinal Pole (dated November 11, 1538) says that his hopes had been wakened anew by the pope's attitude. He and his friends thought that all would have been done when the abuses in church life had been put away. This was the judgment of a diplomat of noble and virtuous nature, reared on the best fruits of antiquity and refined through the Gospel, urged on by a desire for peace, and unfettered by dogmatic formulas.
But he was soon to see the other side. In the year 1541 he was papal delegate at the diet and religious debate at Ratisbon. There everything was unfavorable; the Catholic states were bitter, the Evangelicals were distant. Contarini's instructions though apparently free were full of papal reservations. But the papal party had gladly sent him, thinking that through him a union in doctrine could be brought about, while the interest of Rome could be attended to later. Though the princes stood aloof, the theologians and the emperor were for peace, so the main articles were put forth in a formula, Evangelical in thought and Catholic in expression. The papal legate had revised the Catholic proposal and assented to the formula agreed upon. All gave their approval, even Eck, though he later regretted it. This did little good, for the Protestants could see in it only Roman cunning; at home the cardinal fared still worse.
His own position is shown in a treatise on justification, composed at Ratisbon, which in essential points is Evangelical, differing only in the omission of the negative side and in being interwoven with the teaching of Aquinas. Meanwhile the papal policy had changed, and Contarini was compelled to follow his leader. He advised the emperor, after the conference had broken up, not to renew it, but to submit everything to the pope.
In a second decision he is even more ultramontane. It is not difficult to reconcile this course of action with his character, for from the beginning Luther repelled him as did the popular movement in Germany. He lived in the belief that a reformation should begin at the head, and his birth, education, and diplomatic career made him view the question rather from the point of polity than of doctrine, and consequently he was willing to mediate here. But the negative side, which had produced the schism, remained unintelligible to him, he could concede only the marriage of the clergy and communion in both elements.
Meanwhile Rome had drifted further into reaction, and he died while legate at Bologna, at a time when the Inquisition had driven many of his friends and fellows in conviction into exile. He was happily spared a decision which perhaps would have been too hard for him, and so he could leave behind him the character of a man who knew the truth and willed the good.