I said in my previous post that it was not clear who or what the Doctor’s faith was in when he muttered or recited something in “The Curse of Fenric” that overcame the vampires. A commenter pointed out that I was wrong, and encouraged me to go back and listen carefully. It is very moving – turn up the volume and listen closely from around the 50 second mark in this clip:

UPDATE: The clip above which I originally shared is no longer available, alas. This was the link to it:

The one below has just the relevant scene, but makes it impossible to hear what the Doctor is saying: