Introduction

      Entering the chapel of Saint Sylvester in the basilica of Santi Quattro Coronati in Rome, one encounters a sequence of frescoes from the year 1246 that speaks to the dual nature of Christian sovereignty in the Middle Ages. The images present a meeting over nine hundred years earlier between Pope Sylvester, the bishop of Rome, and Constantine, the Roman emperor. Although this encounter never actually happened, medieval Europeans generally believed that that it did. The paintings commence with scenes of the pagan ruler’s leprosy and refusal to bathe in the blood of slaughtered infants, a cure recommended to him by pagan priests, followed by his vision of the apostles Peter and Paul. The two saints instruct him to seek out Sylvester, who was hiding on the outskirts of Rome due to the imperial persecution of the Christian church at the time. The pope meets and baptizes the stricken ruler, healing him of his affliction. Out of gratitude, the emperor hands over his tiara to Sylvester, denoting his surrender of the Western empire to the Roman pontiff. A concluding scene shows Sylvester on horseback while Constantine humbly acts as his groom. By visualizing the past in this manner, the frescoes communicate a message about the proper relationship between the “two powers,” that is, between the spiritual authority of priests and the temporal might of secular rulers, embodied above all by popes and emperors. As the example of Constantine and Sylvester revealed, emperors should ultimately defer to popes, recognizing their superior form of sacerdotal sovereignty.¹

      Outside the walls of the chapel, when its frescoes were new, Christians faced a far more turbulent relationship between the chief representatives of the two powers: a violent division between Pope Innocent IV and the Hohenstaufen emperor, Frederick II. Contention between the papacy and the imperial ruler had begun under Innocent’s predecessor, Gregory IX, who excommunicated Frederick not once, but twice: for the first time in 1227, when the emperor failed to depart on crusade by an agreed-upon deadline, and again in 1239 (nine years after their previous reconciliation), when he was accused of various