Most modern interest in Celestine V has focused on his resignation. He was the first pope to formalize the resignation process and is often said to have been the first to resign. In fact he was preceded in this by Pontian (235), John XVIII (1009), Benedict IX (1045), and Gregory VI (1046). As noted above, Celestine's own decision was brought about by mild pressure from the Church establishment. His reinstitution of Gregory X's conclave system established by the papal bull "Ubi periculum" has been respected ever since.