Possible Clues:

• Canonized fifth-cen. pope
• Fifth-century pope
• Canonized fifth-century pope
• Canonized pope known as "the Great"
• Fifth cen. pope called "The Great"
• "Great" pope of 440-61
• First pope to be called 'the Great'
• Canonized pope who kept 26-Across from invading Rome
• Pope in Attila's time
• Florida University named for a pope