Rowan says in End Of Term that for quite long stretches she believes, so conceivably she might have entered the long stretch of 'not' that she's in at that point at some point earlier in the canon, or left it at some point later. I also have a very dim remembrance of Esther being asked if she believes and unguardedly saying 'I just wish I didn't have to - ' but can't find it in either End Of Term or Attic Term, so possibly she was talking about someone else altogether or I just made it up.

Other than that, AF's believing Anglicans do seem to be the very minor characters - the Jean Bakers and Jess Geddesses - and Jess may well be Church of Scotland or some other variety of Presbyterian, in any case.

As far as Lawrie goes, I think her not finding it credible that anyone else can have faith is all of a piece with her not believing that anyone else has feelings, which is also mentioned in End Of Term. It's not a matter of rational or irrational, so much as the only division Lawrie really respects being the one between Lawrence S. Marlow and the vague shambling shapes that make up the rest of the universe, who only really come into focus when defined by being fair or unfair to Lawrie.