LJ eats the comment once again! (OK, I decided to interrupt to clean fluff from the laptop keyboard).

My own reading is that it's proof of Bunter 'on the job'. The hymn-book neither causes him to believe Hannah's word without question, nor to restrain himself from pushing the personal connection to worm any secret out of it. So when he says that Hannah is "a sincerely religious woman" (and chapel at that) we can genuinely believe that she's telling the truth, because Bunter isn't taking this at face value, he's tested it and found it to be the case.