The notion of sin is also present

another note: Snow and Regina wore black at Leopold's funeral, which is a largely Christian custom although some other cultures have adapted it

The whole idea of sin also ties into the Underworld arc, with the idea of the dead being judged like Cora and Liam and going to the better place or worse place.

Not really a Christian thing. More of a Roman thing, but it's across various cultures as well.  Medieval Europe was similar at times with Asian society and wore white.

Same with the concept of "darkness", wrong, or "sin" in regards to an afterlife. If anything, the UW is heavily Greco-Egyptian.

• When Merlin and Adda find the Holy Grail, they refer to divine beings as gods. Seen the surroundings, they live in the desert and possibly believe in polytheism. Possible religions for Camelot could be Christianity, since in King Arthur and the Knights of the Round Table, Guinevere is sent to an abbey after her affairs with Lancelot. There could also be Egyptian mythology, since the episode scene about the Holy Grail features a desert, or Celtic mythology, since it is near DunBroch and in The Bear King, they have the Ale of Seonaidh, which is a Celtic myth. ("Nimue"), ("The Bear King")
• Hercules is a Greek demigod son of Zeus and lives in the Enchanted Forest, which means there is Greek polytheism there. Also, Hades is a Greek god. ("Labor of Love"), ("Souls of the Departed")
• Ursula seems to be an original goddess and a part of her own pantheon. I believe that Maritime believes in both Christianity and has their own pantheon. ("Ariel")
• Norse, since Arendelle is a very Norse place and possibly has people who believe in the Norse polytheistic faith, and Gerda shares a name with a Norse frost giant goddess. ("The Snow Queen")
• List of Real-Life Pantheons
  • Greek
  • Egyptian
  • Roman
  • Norse
  • Chinese/Japanese
  • Hindu
  • Celtic
  • Aztec
  • Native American
  • African
• Native American belief has not been shown, but is commonly associated with forests and people who live there. So Red Riding Hood, Baelfire, and other characters who live in the heart of the forest might believe in Native American belief, but is not that likely, since Red is shown to have belief in Christianity a little. But it could be possible.
• There is a cross on the Dark Grail^[1], at the Temple of Prometheus^[2], and on a few gravestones in the Underworld.
• Those are real-life pantheons. So far, there is Greek/Roman and Celtic. Possibly Egyptian, Roman, Norse, and Chinese/Japanese (Mulan's Village must believe in Chinese or Japanese faith, since they have the symbols over their village.) Hinduism and African mythology has not been seen. There is also Christianity. So, basically, different parts of the Enchanted Forest believe in different gods. There is no official religion, but most people in the Enchanted Forest believe in Christianity.
• Please note that Ursula the goddess is a totally original goddess and a part of a bigger pantheon. Maybe there are original religions with new deities and stuff, because in "Ariel", Ursula is not a part of a particular pantheon. ("Ariel")

Honestly, I look at all of the stories of OUAT as Esoteric. PERFECT example: 

http://www.stillnessinthestorm.com/2014/11/decoding-fiction-science-of-meaning-as.html

When I saw Merlin's Origin, I knew that they were not attempting to give some sort of in-universe account of his journey (not like we think of it, conventionally), but rather the story of Alchemy. 

What's interesting to me is that all Magic in the OUAT universe originated from the Holy Grail, the most Sacred Cup IRL. Why on Earth would they throw that in there if they beleived in Polytheism? And it's not like a passing object, it's the only Item that can control the Dark One/Dark Magic(k). In a sense, Light Magic(k) was Born from Will (Merlin's Holy Quest), and Dark Magic(k) is born from Light Magic(k). Which if that makes sense, then look at the relationship of: Light Magick is born from Will (a Man's Will), and Dark Magic(k) is born from Light Magic(k), which becomes Female (the Mother). So: you have Merlin (Man) as Light Magic(k), revealing Dark Magic(k) in Niumbe (Female): the interplay of the Dualisms: Father being overthrown by his Children, Son's taking their father's place, Male and Female, True Love's Kiss (male/female) breaking all Magic(k).. 

I think to really appreciate the subtext of OUAT means to understand some Esoericism, which is really interesting to me  

Snow's marriage was realized by a bishop , James fights a man named Behemoth, there is a cross on Daniel's grave , in Skin Deep Regina lies and say that clerics had to clean Belle's soul with scourges and flaying after she come back home from Rumple, Lancelot's nickname is Leviathan, there is a picture of an angel in the marketplace, Mr. Gold mentions fire and brimstone, that are classical simbols of God's wrath in the bible, in the episode Mother Regina mentions a church, Arthur mentions the burning bush, Violet's horse name is Nicodemus, Hades mentions the Devil, and there's the Holy Grail.

But still there reference to multiple gods, primarily, the Greek ones.

I believe that the religion of Fairy Tale Land, or at least of the Enchanted Forest is a polytheistic version of Christianity.

"In the beggining Kronos created the Heaven and the Earth"
"And Zeus, Hades and Poseidon decided to created the man in their holy image."
"And then Poseidon opened the Red Sea to the israelites pass by."
"And Zeus came down from the Heavens and take the form of a mortal man, Jesus Christ"
etc.

I loved this theory!

Storybrooke has nuns (I thought that was a religious thing)

Storybrooke might be different, because it was all part of the curse. Since they were in the LWM, the curse likely did away with all the EF religioons