You can expect certain manners from your peers. Denying God is bad manners. I would also note the guidelines on reviewing a vulgarity message:
Third is behaviour, this is mostly for roleplays. A noble is
first and foremost a noble. No matter if he bloodies his sword in the
bodies of his enemies or poisons the wine of the ruler, his actions and
his way of acting set him apart from the commoner in a hard-to-describe
but easy-to-spot way.
Your actions and way of acting can be vulgar. Atheism is not the sort of thing a noble could ever espouse in a polite way.
But more importantly, I don't think I, or anyone, has to be able to express exactly what's vulgar about a message. Note that the vulgarity pages repeatedly state how hard vulgarity is to define! Anybody thinking vulgarity is easy to define is also out of line with those pages. Now, I'm fine with that, because I also disagree with them: I don't think vulgarity is easy-to-spot. I think there are plenty of cases where reasonable people, even reasonable people with similar definitions, can disagree about if a thing is vulgar.
Which is why I think if you have a pretty reasonable idea that you think something is vulgar, even if you can't give a 4 point detailed case for why it is, it's fine to report it. I don't think Athena's message was some perfect textbook case of vulgarity. Far from it. I'm not shocked other players agreed; but I wouldn't have been shocked if they'd disagreed. Because I know it's a borderline case under many definitions of vulgarity (not my preferred definition, as it were, but I know mine is a minority position).
In terms of reasons I thought the specific message was vulgar, I'll direct you to my first post in the thread. It was not exclusively about atheism, and had a great deal to do with the context of the message.
But I'll reiterate what I've said many times: erroneous reporting, even
frequent erroneous reporting,
is not abuse. The game has a built-in mechanism for dealing with erroneous reporting (which I think we are all agreed should be enhanced), which implies the possibility of legitimate error.