How much do mortals (ie, the people in Creation, leaders and peasants alike) know about Erebusian Mythology? I hope to The One (does Creation know about Him?) they;re not missing out on this
One of my favorite subjects.
The One: I would wager that, from the viewpoint of a mortal, suggesting the existence of the One would be met (quite reasonably) with skepticism. The Luonnotar worship him (though, unless they get around to working on that altar, that's not much better then worshiping Arwan for all the good it does them), and I'm sure scholars would suggest such an entity as equivalent to Aristotle's First Cause. That said, they would probably imagine such an entity as more of a force of nature than a god to be worshiped. Cassiel knows about him, and came to the assumption that he does not seek worship, and therefore sees no reason to spread- the word. (His bit in the 'pedia is a rare exception to this. It has been implied, probably in jest but maybe not, that he wasn't entirely sober at the time.) The gods have no reason to tell the people of the One; even those who don't oppose him wouldn't want to lose followers that could be used to defeat their foes.
Creation: Many religious sects would likely go so far as to declare their own god the Creator; the Ashen Veil and Overlords would certainly go for such, and I can see Ceridwen doing the same. In my file of unfinished short stories, I've started a commentary (written from in-universe perspective) on the holy books of the Ashen Veil... I might get around to finishing it later, but it basically posits that the Ashen Veil believes Agares was the original god, and it is the good and neutral gods who rebelled against him out of jealousy of his glory, seeking to obscure his nature to the people of Erebus. I've not thought much about the other religions, though. Kilmorph's followers might not be above giving her credit for the creation of Erebus (which is 1/21 true), seeing that they place such high value on building things. The Empyrean would likely argue like modern philosophers about First Causes and the like, but seem unlikely to say Lugus himself created Erebus. The Overlords' followers would have approximately as many creation stories as followers, though many would likely suggest Erebus started as nothing but water. Esus' followers, in symmetry to their sun-worshiping nemeses, probably wouldn't bother calling their god the creator, but in their case, they simply wouldn't care how Erebus started. The Fellowship would likely assume Erebus eternal in both directions, or possibly take a page from Hindu beliefs and believe it simply one part of a cycle of perpetual creation and destruction. Frankly, though, I have no idea regarding the Order.
Gods: As to knowing all 21? I'm sure any very bright scholar with plenty of background research could figure out which 21 gods were real, though not necessarily prove it. Erebus is populated with minor cults worshipping non-existent gods, or other entities posing as gods, though. (Technically the Overlords might be the largest, since they're basically manifest nightmares and not truly gods... though they make a damn good case. On the other hand, Falamar refers to the Overlord's believers as worshiping Danalin, though, so they may simply assume the Overlords his agents.) The hardest of the gods to determine would be Ceridwen (Goddess of Secrets for a reason), Esus (God of Deception) and Mammon (who likes his followers in the dark). But considering numerology is practically a hard science on Erebus, at the very least they could figure out the number of real gods and then cross-reference with the sources of mana (EDIT: and/or important astrological signs), and finally figure out which gods go where, and end up with their doctoral thesis in theology. They might make a few errors (I can see them writing "Overlords" by Mind, "???" by Shadow, and "N/A" by Ice, assuming Mulcarn permanently removed and Auric a madman if he's not near ascension) but get the basics about right.
Hell: The existence of Esus' hell would be an especially well-kept secret, but Mammon would jealously guard the dark secret of his own vault as well; I doubt even those within would realize where they were. Camulos would probably paint his vault as Valhalla. The existence of Mulcarn's mountainous home would be well known, where his loyalest followers are preserved below, but not the swamps below. I'm not going to even guess regarding Ceridwen. Aeron's followers would know plenty about his vault that is technically true, but not that it's part of the greater machine. And Agares... by the time you're in Agares' vault, it's usually way too late to escape. Unless you happen to be a total badass with divine aid...
EDIT: *Waits for MC to correct his knowledge of Erebusian lore with fury to match his Freshman English professor.*