Possible Clues:

• Pope during WWII
• One of a holy 12
• Appropriate-sounding papal name
• Last Pope to be sainted
• Name of twelve Popes
• One of a dozen popes
• Apt-sounding papal name
• Eugenio Pacelli, he was Pope -- XII from 1939 to 1958
• 20's-50's papal name
• One of twelve popes, or an antipope