Giovanni Angelo Medici was born on March 31, 1499 at Milan to the noble Milanese Medici di Marignano family (not related to the famous Florentine Medici). Elected Pope on December 25, 1559 after a four-month conclave, taking the name Pius IV, his six-year pontificate completed the Council of Trent and institutionally established the post-Tridentine Catholic Church.
Pius IV reopened the Council of Trent on January 18, 1562 — the Council's third session (1562–1563). The Council addressed the remaining doctrinal-and-disciplinary questions and concluded on December 4, 1563 — after 18 years of intermittent sessions.
The Tridentine decrees were solemnly confirmed by Pius IV's bull *Benedictus Deus* on January 26, 1564.
Pius IV issued the Tridentine Profession of Faith on November 13, 1564 (the standard Catholic-confessional formula for reception into the Catholic Church), the Index of Prohibited Books (Tridentine Index, 1564), and founded the Roman Catechism (the Catechism of the Council of Trent, published 1566 under Pius V).
He commissioned Michelangelo to convert the Roman Baths of Diocletian into the Basilica of Santa Maria degli Angeli e dei Martiri (1561), where Pius IV himself was buried.
He personally appointed his nephew St. Charles Borromeo (see his existing entry) as Cardinal and Archbishop of Milan — producing the institutional relationship between the Tridentine papacy and the Borromean-Milanese Catholic Reform.
He died at Rome on December 9, 1565.