I don't think of Nicola as being particularly ignorant, just maybe not terribly interested (she says at one point she loathes Scripture lessons). In End of Term it seems she's almost more interested in observing other people's religious beliefs (Patrick's, Jess's, Ann's, Miranda's, Dr Herrick's) than interested in the subject herself. And she tells Miranda at the end of the play that she only paid attention when Lawrie was acting (a complete contrast to Patrick and Miranda's responses) and although of course this is partly because she is singing, I think it speaks to her character too.