No, St. Celestine V was not the first pope to resign. Many historians of the Roman Catholic Church think that the first pope to abdicate was St. Pontian, who reigned from 230 to 235. Celestine V reigned from July 5 to December 13, 1294. Other popes who resigned include Gregory XII (reigned from 1406 to 1415) and Benedict XVI (reigned from 2005 to 2013).