Which Other Popes Have Resigned?
UPDATE, 2-28-13: As of 2:28 pm today, Pope Benedict XVI has stepped down from the papacy.
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In an unexpected announcement today, Pope Benedict XVI stated he is resigning from the papacy as of February 28. Benedict's abdication, reportedly due to ill health, apparently took even the pope's closest advisors by surprise. Indeed, a pope hasn't stepped down from the papacy in over six hundred years, and the few instances when popes have resigned have been for reasons either more political -- or more corrupt -- than health.
A look back at the confirmed instances of papal abdication:
Benedict IX, resigned 1045 -- Benedict IX was one of those early popes who would be completely unrecognizable to us today -- contemporary writings refer to his "many vile adulteries and murders." Born into a powerful political family in Rome (two of his uncles had been pope before him), Benedict IX first became pope in 1032, while still merely a teenager. He was forced out of Rome by his opponents on several different occasions, first in 1036 (he was restored to the papacy by the Holy Roman Emperor), then again in 1044 (this time, Benedict IX used his own troops to back his claim). In 1045, the Roman priest John Gratian (who would become pope himself for a brief time, as Gregory VI) actually bribed Benedict IX to resign the papacy, but Benedict had buyer's remorse and returned with his troops to Rome in 1046. He struggled with various challengers until finally being forced out for good in 1048. Benedict IX also holds the distinction of being one of the few pontiffs to have actually been excommunicated for his misdeeds.
Celestine V, resigned 1294 -- Celestine V was a Sicilian monk who was temperamentally the exact opposite of Benedict IX. A deeply pious man who spent most of his life as an ascetic, he was elected pope in 1294 at the age of 79 on the basis of an angry letter he sent to the College of Cardinals upbraiding them for their delay in making a selection! Celestine wasn't happy about his election, and tried to avoid taking office until forced to by a delegation from the cardinals. Celestine's most important papal bull decreed the right of the pope to resign the papacy, which he then promptly did a scant four months after assuming office. (For this act, Dante consigned him to hell for cowardice in The Divine Comedy, though Petrarch was more sympathetic).
Gregory XII, resigned 1415 -- Gregory XII abdicated the papacy not because he was corrupt, nor because he was a cranky old hermit. He abdicated for political reasons, to end a schism within Catholicism that had been festering for nearly forty years. First, some background: From 1309 to 1377, the popes resided not in Rome (problematic, considering that the pope is Bishop of Rome), but in Avignon, France due to a dispute with the French king. Pope Gregory XI returned the papacy to Rome in 1377, and after his death a year later two different popes were declared by rival groups of cardinals: one reigned in Rome, the other in Avignon, and both were backed by different Catholic powers (the former by the Holy Roman Empire, as well as England and most of the Italian city-states; the latter by France and the Spanish kingdoms). This is referred to in the textbooks as the Western Schism. Roman Pope Gregory XII, along with his rival in Avignon, John XXIII (who is charmingly referred to as an "antipope" in Catholic histories) agreed to both abdicate their papacies to allow the College of Cardinals to elect a new, unified pope. Though the struggle to reunify the Church continued for two more years, Gregory XII abdicated in 1415 and spent the rest of his life in obscurity. He was, until now, the latest pontiff to abdicate from the papacy.
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Considering that Catholic tradition holds that the first pope was Peter, the disciple of Jesus -- and that Christianity spent the next three hundred years in various states of repression, acceptance, and officialdom within the Roman Empire before Constantine the Great became the first Roman emperor to convert to Christianity in the early 300s -- it's not surprising that the historical record of Roman Empire-era popes is spotty. Still, Catholic tradition holds that at least two popes (possibly more) ended their reigns in ambiguous circumstances, which perhaps involved their resignations. An apparently lost chronicle (which formed the basis of another, surviving chronicle from the era -- so it goes with pre-modern history) records that Pope Pontian (230-235), who was pontiff during the reign of Roman emperors Severus Alexander and Maximinus Thrax, abdicated after being arrested on the orders of Maximinus and exiled to Sardinia. Pope Marcellinus (296-304?) ended his papacy in uncertain circumstances during the Emperor Diocletian's persecutions of Christianity -- some sources say he was martyred, others that he renounced Christianity and embraced the pagan Roman gods.
UPDATE: Charles Keenan, a PhD candidate at Northwestern University who specializes in Catholic history, also mentions Silverius (536-537) and John XVIII as resigned popes in an article he wrote for HNN. Silverius was the son of a previous pope (he was conceived before his father became a priest) and was deposed by the Byzantine empress Theodosia -- Keenan writes that she forced Silverius to resign, then installed her own candidate as pontiff. The reign of John XVIII (1004-1009) is poorly documented, but at least one source says that he died as a monk.
Further Reading: