I said this in the fav/least fav threads: it’s a cup, with crosses, in Arthurian mythology/with Arthurian characters, and being called The Holy Grail. All that combined means it’s the Holy Grail. There aren’t multiple Holy Grails. It’s a unique item. They aren’t making a unique interpretation; they are taking VAST liberties with a relic in order to force it into the narrative because everyone knows that the Grail is *the* quest object in Arthurian mythology.
If they wanted just some Greek/polytheistic relic to bestow immortality, have Merlin drink some Ambrosia/Nectar from an sort of cup. Easy peasy. We know Greek gods exist in this world. We’ve established that and it’s a nice way to combine the Arthurian mythology and Greek myth, which we’ve been doing all season and is what you’re actually suggesting they are doing here–except for the the object, the narrative, and the situations reads.
Having THE Holy Grail (whatever “unique interpretation” you want to fan wank, it was very clearly supposed to be THE Grail) is super problematic because it raises so many FREAKING questions and issues. I have no problem with there being a magical cup in OUAT and, more specifically, in Arthurian mythology, but to call it THE Holy Grail is incredibly problematic because it suggests that Jesus existed in and had the same influence on the Enchanted Forest as he did in our very real world.
Christianity is not sui generis; it did not form in a vacuum completely devoid of outside forces and therefore could equally form in another universe. What outside forces you ask? Judaism, for one. Polytheism for another. History between those two forces, for a third. All of that (and so much more) led to the rise of apocalyptic Judaism and, eventually, to a preacher named Jesus who roamed around ancient Galilee, trying to tell people that times were ending. This is to say nothing of the apostle Paul who is responsible for Christianity going out to the Gentiles where it really took root (Jewish-Christianity fading away as the city of Jerusalem burned). If any of that–Judaism, Greco-Roman culture, the interaction between the two, and Paul–happened in the Enchanted Forest, then we’ve never seen it nor heard of it. And, like I keep saying, Jesus was not a realm jumper.
I think this goes into the further complicated question of why FTL and these other realms exist at all in the OUAT universe. Why were the events of these realms unfolding many years after our own versions of the stories were written? Why did the Darlings live in the real Land Without Magic London instead of one of the “Fictional Londons”? Why are historical characters like Blackbeard in FTL? Mulan and Robin Hood are considered legends that could be based on real historical people, but there’s no doubt that Blackbeard (Edward Teach) was a very real person. Why do these realms just so happen to share the same spoken languages that exist in our world? Just like you mentioned with religion, each language “did not form in a vacuum completely devoid of outside forces.”
It’s probably way too deep for this show to ever explore, when the real world continues to question and debate the meaning of life. The closest answer we’ll likely ever get was when Isaac talked about these worlds as simply “realms of storytelling.” We’re both thinking deeper about this than the writers clearly are. *head explodes*