The History of the Popes (Vol. XIV) : From the Close of the Middle Ages discusses the pontificates of Marcellus II (1555) and Paul IV (1555–1559) during the mid-16th century. This volume focuses on aggressive church reform, the geopolitical conflict between the Holy See and Spain, and the spread of Protestantism. Drawing on primary sources from the Vatican Secret Archives, the volume details the brief one-month reign of Pope Marcellus II—highlighting his strict economic austerity measures and his steadfast commitment to the Council of Trent and Church reform—and chronicles the tenure of Pope Paul IV (Gian Pietro Carafa), specifically his harsh measures to eradicate heresy, his efforts to reform the College of Cardinals, and his deep involvement in the Catholic Counter-Reformation; it also explores the political tensions and power struggles between the Holy See and the Spanish Empire, as well as the Church's reaction to the Protestantism taking root in Eastern Europe and the Low Countries. This comprehensive work spans 40 volumes, chronicling the reigns of 56 popes.