In 1302, Pope Boniface VIII issued the bull Unam sanctam, proclaiming with extraordinary confidence that "it is altogether necessary to salvation for every human creature to be subject to the Roman pontiff" (Boniface VIII, Unam sanctam, 1302). Within a year, the aged pope lay dead following a humiliating assault at Anagni by agents of the French king, his grandiose claims to universal authority shattered against the hard reality of rising national monarchies. This stark reversal captures the essential drama of the late medieval papacy: an institution oscillating between triumphant assertions of spiritual and temporal supremacy and devastating crises that exposed the fragility of its power. The three centuries between the pontificate of Innocent III and the eve of the Protestant Reformation witnessed the papacy's transformation from what appeared to be the apex of medieval Christendom's unity to an institution whose corruption and worldliness would provoke the greatest rupture in Western Christian history.

This study draws on diverse primary sources to reconstruct the late medieval papacy's complex history. Papal bulls, letters, and official pronouncements reveal how popes understood and articulated their authority. Chronicles and narrative histories, from monastic annals to humanist histories, provide contemporary perspectives on papal actions and reputations. The records of church councils, from Lateran to Constance to Basel, document debates over ecclesiastical governance and reform. Financial records illuminate the fiscal machinery that supported papal government. Literary sources, from Dante's poetry to Petrarch's invective to Erasmus's satire, capture how educated Europeans perceived papal corruption and worldliness. Letters from mystics like Catherine of Siena and Bridget of Sweden offer spiritual critiques distinct from intellectual or political opposition. Theological treatises debating papal authority, from defenders like Giles of Rome to critics like Marsilio of Padua and William of Ockham, reveal the intellectual stakes in conflicts over papal power. These diverse sources, read together, provide a multifaceted portrait of an institution navigating unprecedented challenges with mixed success.