“Great, so are we going to guess at the sign? Each wear a different one and hope we get it right? Or, and my personal favorite, go home, have a beer and tell them everything is where it’s supposed to be?”
"I suggest we check out the rest of the place then and see if we can find something that might be considered a ‘Sign of the Prophet’.
"I also suggest that no one touch the book until we have this ‘sign’. After all, we don’t want to accidentally stumble into something we’re not ready for. Does anyone see any mechanism that would trigger a secret door?
"By the way, do any of the more religious amongst us have a clue what prophet is being referred to? I don’t have any idea.
Melon leans on random objects throughout the room. That’s usually how characters in stories find hidden levers.
It said, ‘to all those who bear the Sign,’ so perhaps we are looking for a tattooed bear.
“Or the sign is an undead bear waiting to eat us.”
“Come on, lets check the rest of the place and see what we find, then we can decide with all the info”
I’m going to stand in the 2nd safest looking spot, because the safest looking spot must obviously be a trap, and I’ll try to scan the room looking for a Sign.
I’ll use any religious proficiency I might have to recognize the sign. I will also go back and look at the markings by the door.
Actually, I guess I’ll follow Wolvie, and try to use intelligence and my arcane knowledge to see if I can succeed where his foolish religion fails.
Ah I see we have a doubter in our midst. Since you don’t believe in the soothing power of rain to bring blessings upon you, next time you need healing I might just be out of Cure spells. Of course you wouldn’t want them anyways, with them being foolish and all.
“I already checked for traps, so you’re good ;)” Right now I just go along with the group, trying my best to look astute and helpful.
It’s a biggish room. Obvious places to check would be the lectern, the book, the murals, the dais, the door you entered by… Eventually someone musters the courage to actually touch the book (which is on the point of disintegration) and finds a strange symbol on the cover of the book and also on the sloping surface of the lectern where the book is placed. It looks rather like a seven-pointed star.
And should I ever achieve my goals of becoming an alchemical master of the poisonous arts, perhaps we shall one day see how well your soothing rains and Gods feel about you in the throes of one of Nature’s poisons. I think you may wish for your handy chemist at that moment, no? Until then, I suppose it’s best for us to remain neutral allies and pool our resources for the betterment of ourselves.
:Ishtar will inspect the book- can he read/understand any of it?
(And I swear if there’s an exploding ruins on it… well maybe I’ll just read it next to Auto to be safe.) 
But yes- I’ll read the book and try to see if I can figure out anything more than “put the book’s symbol onto the slot” If nothing happens, I’ll reserve it up to the judgement of the others whether I should just put the book down on the lectern thingie. But in the meanwhile, lets try to get some more info.
“It says no harm will come to those who have the sign, so why don’t we take the book with us and go on our way deeper into the catacombs?”
“And painting the seven pointed star in our clothes could be a good idea too”
“Did any one bring any paint or inks? Otherwise I want whoever is carrying that book to be upfront with me.”
The book is on the point of disintegration, but I believe some of the literate types have writing materials.
I’ll check my pockets and see if I can try to copy down the symbol onto a parchment or such thing… (I’ll also try to inconspicuously see if I notice if this book is a spell book per chance or may contain any spells I ought to try to learn? I might want to try to copy down more than just that symbol if it’s worth my time…)
If I can’t find my writing tools, I could just use the ash/soot from a torch and try to smear that symbol on someone- **Auto **could be my first attempt. (Joking 
)
“Hey that symbol makes us safe right? Smear away.”