Possible Clues:

• Start of a string of 13 popes
• Sainted pope after Sixtus III
• First of a succession of 13
• Sainted emperor called the Thracian
• Pope who kept Attila the Hun at bay
• Pope before Hilarius
• Predecessor of Pope Hilarius
• Early pope called 'The Great'
• Pope who repelled Attila the Hun