Pius IV (1499-1565), by backing the Council of Trent in its last and extremely tense period, emerged as one of the great popes of the Catholic Reformation. By his temperate and tractable approach, he broke with the severe regime of his predecessor, Paul IV.

Giovanni Angelo de' Medici, who became Pius IV, was born into the lesser nobility of Milan on March 31, 1499. His family was not related to the famous Medici of Florence. He received his early education at Pavia, and in 1525 he earned a doctorate in canon and civil law at the University of Bologna. The next year Medici began his service in the Church as a protonotary apostolic. Under Pope Paul III he gained a breadth of experience in administration within the papal states and in diplomacy on missions to Hungary and Transylvania. At the age of 46 Medici was ordained a priest. The same year, 1545, Paul...