How do I save Rome from Attila the Hun?

Hint 1 of 13:

Drop by the Vatican and bring Pope Leo with you.

Hint 2 of 13:

You need to convince him that God is powerful.

Hint 3 of 13:

Find a way to create a minor miracle.

Hint 4 of 13:

Attila doesn't know about the future, but you do.

Hint 5 of 13:

The needed "technology" isn't too far ahead in the future.