Religion

In the most of shown parts of the world of One Piece there are no apparent signs of religion, and no god-like beings have been introduced as yet. However, in some areas, e.g. Skypiea, a strong religious sentiment is common. Also, many minor religious references are scattered across the series.

God of Skypiea
In Skypiea, the title of "God" refers to the ruling body of the sky islands. God is generally looked at as a governing class of sorts. However, other than the title itself, the position of God holds no religious or divine overtones, being a regular physical being who is neither all-powerful nor perfect. The first known man to hold this position was the unnamed God of Skypiea, who governed four hundred years prior to the current storyline, and Gan Fall was the previous God eight years prior, before Enel took over. Enel took this title too seriously, matching some of the "all powerful" and "omniscient" traits prior to his defeat by Luffy. After Enel was gone, Gan Fall reclaimed the title.

"Gods" of Shandora
Shandians (at the times when they lived on Jaya) worshipped giant pythons (e.g. Kashigami). They even made human sacrifices to them. This practice ceased when Montblanc Noland and his crew arrived at Jaya and cured the tribe of Tree Fever.

"Gods" of Alabasta
In Alabasta, the title "God" refers to the strongest guardians of the royal family. Pell and Chaka (the two head guards under Igaram of the Alabasta Kingdom) often refer to themselves as the "Guardian Gods" of Alabasta and the royal family. Also, their Zoan-type Devil Fruit transformations (a falcon and jackal respectively) resemble the two Egyptian gods Ra and Anubis, the two strongest gods in the Egyptian pantheon.

The Satanic Cult of Namakura Island
The satanic cult indicates that there might be a religious belief in the Devil in the One Piece world. The cultists on the island were trying to summon Satan, another name for the Devil, but instead ended up 'summoning' Brook.

Other mentions of God and religion
In the Arlong Arc, a cross marks the grave of Bellemere.

In the Whiskey Peak Arc, some of the Baroque Works mercenaries were dresses as Wikipedia:Nun#Christianity, one of them using some kind of gas hidden in what appears to be a christian cross.

Sanji has made many references to a possible God. In Loguetown, after lightning struck the scaffold Luffy was on and saved him from Buggy (calling the event "Divine Intervention" in the Funimation dub), Sanji asked Zoro of his beliefs, though Zoro didn't answer. During the Skypiea Arc, Zoro claimed he never prays to God and doesn't care whether one exists or not. After defeating Jyabura of the CP9, Sanji mentions God again, saying "God creates food, and the Devil creates seasonings".

At one point, Conis questions whether or not God exists, she then shouts out "Help us, God!!". However, it is more likely that she had been referring to Gan Fall.

There is also a reference to Davy Jones as a kind of god. When pirates have a Davy Back Fight they have to throw three coins into the ocean as an offering to Davy Jones, so that the 'sacred games' may begin.

There is also a reference to sea gods. Before entering the Florian Triangle, the crew picked up a barrel with the sign saying "Sea god's Houzen" (former treasure). It was actually a trap from Thriller Bark. However it seems that some sailors do put alcohol and reserves into floating barrels as an offering to the guardian deities of the sea. It seems to be customary that if someone picks one up, they may have the contents, though they will need to replace the 'offerings' and put the barrel back into the sea. After the Strawhats picked up the floating barrel, Luffy prayed to 'God' for some tasty snacks, which Zoro finds ironic since his captain was the one who beat "God" in Skypiea. King Neptune has also been refered to as God by several people.

Bartholomew Kuma has a bible (notable as it has "BIBLE" in full capitals) in his hands and always carries it with him. Gekko Moriah wears a cross around his neck, Mihawk also has weapons modeled after crucifixes, so one could say the Shichibukai might be a holy organization. However, the religious emblems they carry could simply be due to the fact that the privateers they are modeled after happened to be religious. On a separate note, while the city of Alabasta looks to be a Middle Eastern land, their graves are cross-shaped. The graves in Fishman Island's Sea Forest are also shaped like crosses.

At Sabaody Archipelago, Caribou addresses a god of some sort, stating "O God, forgive him!" and referring to a marine soldier as "sinful".

The Flying Dutchman and its captain, Vander Decken, were said to be cursed by God for the captain's atrocious acts after suddenly going deranged and killing his crew one stormy night, as well as spitting at God. They were cursed forever to sail the ocean, never able to make landfall.

Many characters in the series have been referred as "demons" or "devils", such as Gin, Nico Robin, Oars, and Dias.

In episode 48 of the anime, the Liturgy of the Hours can be heard in Logutown when Luffy sees the sign for the Gold Roger bar.

Two towns near Water 7, St. Poplar and San Faldo, both contain the word "saint" in their names ("san" means saint in Spanish and Italian), suggesting there is a belief in saints or angels in One Piece.

Trivia

 * In Oda's 1993 one-shot God's Gift for the Future prior to One Piece, "God" appears as a comical old man who isn't omnipotent at all.