become pope Pope Paul III, Cervini was appointed a papal secretary (1534–49) and served as a close advisor to the pope's nephew Alessandro Farnese. He was made a papal protonotary. He travelled in the suite of the Pope during the papal visit to Nice, where Paul III was promoting a truce between Francis I and Charles V. He then accompanied the young Cardinal Farnese on a journey to Spain, France and the Spanish Netherlands to help implement the terms of the truce. Paul III later appointed him Bishop of Nicastro in 1539. Cervini was not, however, consecrated bishop until the day he himself was elected pope. On 19 December 1539, while Cervini was still on the embassy to the Netherlands, Paul III created him Cardinal-Priest of Santa Croce in Gerusalemme. When, almost immediately afterwards, Cardinal Farnese was recalled to Rome, Cervini stayed on in Spain as nuncio. Over the course of the next decade Cervini also became the apostolic administrator of the dioceses of Reggio and Gubbio. His house in Rome became a center of Renaissance culture, and he himself corresponded with most of the leading humanists. During the Council of Trent he was elected one of the council's three presidents, along with fellow cardinals Reginald Pole and Giovanni Maria Ciocchi del Monte (the future Pope Julius III). He continued to serve in that role throughout the remainder of Paul III's papacy, after which he was replaced to placate the Holy Roman Emperor Charles V (1519–56). He was credited with defending not only orthodoxy and Church discipline, but also the universal claims of the Papacy in spiritual and temporal affairs, and with such vigor that the Emperor was affronted. In 1548 (or 1550) Cervini was placed in charge of the Vatican Library, with the title of Protettore della Biblioteca Apostolica. The Apostolic Brief of his appointment, however, came from the new pope, Julius III, on 24 May 1550, and in it he was named not Vatican Librarian, but Bibliothecarius Sanctae Romanae Ecclesiae because he was the first cardinal to be placed in charge of the library. During his administration, he employed the services of Marcello and Sirleto, as well as Onofrio Panvinio (who was especially consulted in matters of Christian archaeology). He added more than 500 codices to the holdings of the Library, including 143 Greek codices, as his own entry book (which still survives as Vaticanus Latinus 3963) testifies.