Antonia Forest fans

I find Lawrie's religious ignorance entirely plausible. Lawrie doesn't seem to be particularly interested in religion or consider it important, so she doesn't pay much attention to it - her classmates under the same impression are presumably likewise. No doubt if they have a fun passage in scripture knowledge* she enjoys reading it out dramatically, but she isn't interested in context. Also, it's pretty clear that religion isn't something that the Marlows or Kingscote talks about a great deal, so no-one picks up on her misunderstanding or provides a context in which she might think more herself.

*Or whatever it is called. I may be thinking of Wooster.