I also feel that pity would be Fanny's primary reaction to Mary's lack of religious seriousness, as you suggest. It seems to me a more 'appropriate' & Christian response than anger.

Your analysis of what moves Fanny to anger here and elsewhere in the novel is very helpful. I see now that it isn't Mary's irreligiousness (is that a word?) which makes Fanny angry, but slurs and attacks on people she cares about. Thanks Robbin - this has bothered me for ages.