Someone I found through who_daily said that the line about the Doctor's faith was a nice callback to the Curse of Fenric, and reading the TARDIS Index File's summary I found this:
What eventually drives them back however, is the Doctor's faith — as he chants the names of his former companions, the haemovores scream and retreat. The Doctor explains that faith forms a psychic barrier that the haemovores cannot penetrate.
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What eventually drives them back however, is the Doctor's faith — as he chants the names of his former companions, the haemovores scream and retreat. The Doctor explains that faith forms a psychic barrier that the haemovores cannot penetrate.