Can a pope resign? Celestine V, Gregory XII and Benedict XVI

Yes, a pope can resign, though it almost never happens. Under canon law, in the Church's 1983 Code, for a papal resignation to be valid it must be made freely and properly manifested, but not that it is accepted by anyone. Because a pope conventionally holds the office for life, a resignation is an uncommon event: before the 21st century, only five popes unambiguously resigned with historical certainty, all between the 10th and 15th centuries. The most recent, Benedict XVI, left office on 28 February 2013.

The best-known early case is Celestine V in 1294. A monk and hermit, he had been pope for only five months when he issued a solemn decree declaring it permissible for a pope to resign, and then did so himself. His formal instrument of renunciation cited "the desire for humility, for a purer life" and the "deficiencies of his own physical strength". His successor Boniface VIII had him imprisoned in a castle, to prevent his installation as antipope, and he died there in 1296. His decree, with Boniface not revoking it, ended any doubt among canonists about the possibility of a valid papal renunciation.

Gregory XII resigned in 1415 to end the Western Schism, when there were three claimants to the papal throne: Gregory XII in Rome, Benedict XIII in Avignon and John XXIII in Pisa. Before resigning, he formally convened the Council of Constance and authorized it to elect his successor.

Benedict XVI's renunciation was announced by the Vatican on the morning of 11 February 2013 and took effect on 28 February. He was the first pope to resign since Gregory XII, and the first to do so on his own initiative since Celestine V. His action was unexpected, since modern popes had held the position from election until death. He cited his advanced age and health, took the title "pope emeritus", and the conclave that followed elected Jorge Mario Bergoglio, who took the name Francis.