Hi TreeTiger
It's be a bit delicate, asking someone to change the salutations they use in a letter I would think, whether your friend is agnostic or not.
Does the person actually write 'in Christ' at the end of the letter as the closing line, in lieu of 'regard'/'yours sincerely' etc?
I have never come across this before, it seems an unusual way to end a letter, especially if it is a business related one.
I have to be honest though, if I were your friend, I'd just leave this go.
Alternatively they could drop a line stating that as they are an agnostic, they would prefer if religious reference were not used in letters addressed to them.
I would think that this might cause offence though, and possibly is a bit over the top to comment on it to the sender.
What do you think yourself?
Nicola