For that matter the likelihood of the family discussing Nicola's sceptism seriously seems slight too! If they were in the habit of religious conversations over the dinner table then I think Lawrie's ignorance might not have been so complete and nor would the others be so surprised about Ann's distress over "See the tender lamb appears, promised from eternal years."
antfan's Doylist reasoning makes most sense for me.
Re: Religion and sitting like bookends
antfan's Doylist reasoning makes most sense for me.