Canon 332 §2 of the 1983 Code of Canon Law explicitly permits a pope to resign; the Canon stating: ‘If it happens that the Roman Pontiff resigns his office, it is required for validity that the resignation is made freely …’ This Canon in essence codified what was believed to have been tacit in Canons 185 and 188 of the 1917 Code. John Paul II’s (#265) 1996 Apostolic Constitution Universi Dominici Gregis which specifies the ‘special laws’ that come into play during a sede vacante, accedes to Canon 332 §2 by acknowledging in its Clause #3 that a sede vacante may occur due to the valid resignation of a pope.
That popes could resign [i.e., abdicate] is a relatively old notion with documented instances to serve as precedence. Way back, on December 10, 1294, Celestine V (#193) issued a bull, Constitutionem felicis recordationem, from Naples [Italy], stating that the protocols for papal elections (including that of holding them within a sequestered conclave) set forth by Gregory X in 1274 would still apply, in no way altered, even in the event that a sede vacante is caused by the abdication of a pope. Three days later, Celestine V, the Godfather of today’s conclaves, abdicated — just 106 days after he had been consecrated as pope. Alas the resignation did not work per the expectations of this pious pope (as will be described later).
Despite this long-term permissibility of papal resignation, the last pope to have resigned was Gregory XII (#206), on July 4, 1415 — and that was in what can only be characterized as exceptional, unlikely-to-ever-be duplicated circumstances. [See below]
Gregory XII is the only pope to have resigned since the 1294 abdication of Celestine V.
In reality, we only know of four (4) popes that have ‘voluntarily’ and legitimately resigned as of the 1st millennium.
In total, we are only aware of six (6) ‘voluntarily’ and legitimate resignations of popes, though even a couple of these are in doubt.
These being, in reverse chronological order:
1. Gregory XII (#206) on July 4, 1415 [to end the Great Western Schism].
2. Celestine V (#193) on December 10, 1294 [hoping to resume being an aesthetic monk].
3. Benedict IX (#146, #148 & #151), his 2nd term, on May 1, 1045 [when he abdicated, per a deed he had set up, handing over the papacy to his godfather, John Grantin (Gregory VI (#149)), reputedly in exchange for 1,450 pounds of gold (~$25M in today’s prices). Though it may not have been as formal as when he relinquished his 2nd term, Benedict IX fled Rome on September 15, 1044 (after nearly 12 years as pope) to escape unrest by the locals at his papacy, thus leaving the Holy See vacant such that it could be claimed by Silvester III (#147) with help from the then powerful Crescentii family.
4. John XVIII (XIX) (#142) in mid-1009 [is thought to have abdicated, shortly before his death, to spend his last days as a monk at the monastery of Rome's San Paolo fuori le Mura [St. Paul's Outside the Walls].
5. Silverius (#58) on November 11, 537 [having previously been deposed and exiled] may have been coerced into abdicating to legitimize the papacy of his successor Vigilius (#59).
6. Pontian (#18) on September 28, 235 [upon been arrested and deported to Sardina, as part of Emperor Thrax's persecution, so as to permit orderly succession in Rome if there was to be a lull in the imperial crackdown].
That is it — as far as we can tell.
Marcellinus (#29), who succumbing to the harsh Diocletian persecution is said to have surrendered copies of the scriptures and burned incense to the pagan gods, is thought to have abdicated. But, there is no credible record of this.
Then there are situations such as with John X (#123), during the Saeculum Obscurum (Dark Ages a.k.a. ‘pornocracy) of the papacy. He was deposed and imprisoned, probably baselessly, in 928. Though Leo VI (#124) was appointed pope in May 928 following the imprisonment of John X, there is no documentation that John resigned. John X died in 929 — mostly likely suffocated with a pillow!)
Pius VII (#252), ignoring the advice of the curia, agreed to be participate in Napoleon I’s coronation in Paris in 1804. Before leaving for Paris is said to have left a letter of resignation at the Vatican in the event that he imprisoned while in France. He wasn’t and the letter must have expired. Ironically, he was to be arrested and detained 5 years later. He, however, did not resign. It is also said that Pius XII (#261), during WW II, had a letter of resignation in place in the event that he was captured.
THE PAUCITY OF RESIGNATIONS
There are three main reasons as to why there have been only 6, maybe even just 5, known papal resignations — though we also know that, at various times, there have been incessant calls for popes to resign. These 3 reasons being:
1/ The bona fide fear of precipitating a schism [i.e., split] within the Church if a faction of the congregation still insists on maintaining communion with the ex-pope. [Celestine V fell foul of this concern.]
2/ Reluctance of popes to succumb to external pressures which might undermine the mystique and invincibility associated with their office.
3/ Partiality of the ‘papal court’ to resort to other options to force papal succession — hence the continued intrigue as to what happened to John Paul I (#264).
[The 2004 book 'The Death of the Popes' contends, albeit as allegations, that 9 popes, including John Paul I may have been poisoned since the abdication of Gregory XII. Benedict XV (#259), who died in 1922, is one of those listed. Though his death was sudden and unexpected most find it hard to believe that anybody could even suspect that his death was anything other than natural -- due to the flu. But, given the twists and turns of papal history, any and all unexpected deaths are viewed with suspicion. Thus, the death of Sixtus 'the Iron Pope' V (#228), in 1590, is also deemed incongruous, in part given his known unpopularity, though he had been afflicted with malaria multiple times.]
THE RESIGNATIONS OF CELESTINE V AND GREGORY XII
Celestine V, canonized in 1313 nineteen years after his abdication, was a reluctant pope. He was not a cardinal, a bishop or even a priest. Instead, he was an ascetic hermit monk, Pietro ‘Angelerio’ [a.k.a. Pietro da Morrone] well known for his devoutness, who had spent much time, in solitary isolation, in a cave [on a mountain named 'Morrrone']. The sede vacante that followed the death of Nicholas IV (#192) in 1292 had lasted 822 days with the cardinals, not confined to a conclave, unable to agree upon who should be the new pope. At that point, the Dean of the College of Cardinals received a letter from ‘Pietro’ warning of divine retribution if the cardinals dallied any longer. [It is now believed that Charles II, the King of Sicily, a pivotal ruler of that time may have encouraged 'Pietro' to send that letter]. The Dean suggested to the other cardinals that ‘Pietro’ should be elected as the new pope. He was; in absentia. He was finally consecrated 55 days later.
Celestine V, who had accepted the papacy with reluctance, had one mission that he wished to accomplish. He, like Gregory X (#185), two decades earlier, wanted to ensure that cardinals would elect new popes without delay. To encourage this Gregory X had instituted his landmark Ubi Periculum constitution which called for conclaves with very little, if any, creature comforts. In less than two years the cardinals had managed to get Ubi Periculum suspended. Hence the lengthy sede vacante prior to the appointment of Celestine V. Within a month of being consecrated pope, Celestine reinstated Ubi Periculum. Soon after he wanted to get back to being a monk. He sought counsel from Cardinal Benedetto Caetani, the senior most cardinal priest and an expert on canon law. Caetani, it would appear quite rightly, affirmed that there were no laws to forbid a voluntary abdication. The pope then asked Caetani to prepare a formal statement of abdication. The pope, in the meantime, issued a third bull, Constitutionem felicis recordationem, from Naples, on December 10, 1294. This bull stated that all of Ubi periculum would still apply, in no way altered, even in the event that a sede vacante is caused by the abdication of a pope, rather than by his death. The pope was determined to make sure that he did not leave any bases uncovered. In hindsight, it does appear that he did do an admirable job in this respect. Three days later he abdicated.
Eleven days after his abdication Benedetto Caetani was elected pope and chose to be Boniface VIII (#194). Once pope, Boniface, fearing the possibility of a schism, would not let Celestine return to his mountain retreat. Instead, he had him imprisoned. He died while a prisoner from an infection in May 1296 [though he had managed to escape once, early on].
At the time Gregory XII resigned there were two other competing ‘popes,’ viz. Benedict (XIII) and John (XXIII) — the latter who had been a genuine sea-faring pirate in his youth. This was the latter stages of the unfortunate Great Western Schism, 1378 to 1417, which came on the heels of the French Avignon Papacy, which had lasted from 1309 to 1378. To begin with there had been two competing popes, one in Rome, the other in Avignon. At the first conclave to be held in Rome after Avignon, a non-cardinal Italian curialist had been elected as a compromise to appease the rampant Roman ‘mob’ that wanted a Roman, or failing that, an Italian pope. This, the last non-cardinal to be elected pope was to be Urban VI (#203). Despite the conclave that elected him being stormed, multiple times, by the Romans, Urban’s election appears to have been legitimate. Gregory XII was the third in the Roman ‘line’ (or discipline) after Urban VI. The first of the competing neo-Avignon popes had been elected six months after Urban VI.
In 1409 a council was held in Pisa [Italy] to end this schism. The Pisa council, rather than succeeding in eliminating one of popes, ended up creating yet another pope — this one associated with Pisa. Hence the three competing popes. The Roman discipline, however, always maintained that they had legitimacy based on the valid fact that Urban VI was the first to be elected.
Finally in 1414 another council was convened in Germany, at Constance, at the behest of the then German king. None of the three competing popes attended. They all knew that the Council of Constance intended to get rid of them all and create a brand new pope. Gregory XII, however, sent two papal legates, one a cardinal, the other a prince. Gregory had insisted that his legates be permitted to officially ‘summon’ the council and preside over it so that the council would be seen as having been given the papal imprimatur. The German king and the prelates present at the council acquiesced since they understood that having papal endorsement, from the supposedly legitimate discipline, would provide credibility and authority to the council. Once the council was in session, under their presidency, the pope’s legates read out Gregory XII statement permitting the legates to tender his resignation. The ex-pope was then immediately created the titular cardinal bishop of the suburbicarian see of Porto and Santa Ruffina. He was to die two years later. The other two ‘popes’ were deposed though Benedict continued to claim his ‘legitimacy’ until his death in Spain in 1423.