Religion

In many of the shown parts of the world of One Piece, there are no apparent signs of religion, and no god-like beings have been introduced as of yet. However, in some areas, notably Skypiea, a strong religious sentiment is common, and 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 held the position of 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, and naming most of his lightning-based attacks after thunder gods of various religions. After Enel was gone, Gan Fall reclaimed the title.

Gods of Shandora
In Shandia (at the times when they lived on Jaya), people worshipped giant pythons (e.g. Kashigami) as their "Gods", and 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. 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
In 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 or Religion

 * In the Arlong Park Arc, a cross marks Bell-mère's grave.
 * In the Whiskey Peak Arc, some of the Baroque Works mercenaries were dressed as nuns, one of them attacking with gas hidden in a Christian cross. Additionally the tombstones on the cactus mountains are cross shaped and at least one building in Whiskey Peak is shown to have crosses at the top of its towers.
 * 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 did not answer. During the Skypiea Arc, Zoro claimed he never prays to God and does not care whether one exists or not. After defeating Jabra of the CP9, Sanji mentions God again, saying "God creates food, and the Devil creates seasonings".
 * Dorry and Broggy mentioned the god of Elbaf.
 * There is also a reference to sea gods. Before entering the Florian Triangle, the crew picked up a barrel with the banner 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 Straw Hats 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.
 * Bartholomew Kuma has a bible (notable as it says "BIBLE" on the cover) in his hands and always carries it with him. Gekko Moriah wears a cross around his neck, and 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 country 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 "Oh God, forgive him!" and referring to a Marine soldier as "sinful", although Caribou may be using this as a twisted justification for murder.
 * 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.
 * Hody Jones and his subordinates believe that they were chosen by the heavens to obliterate the human race and to change the entire culture of the Fishmen race when they got their hands on the Energy Steroids.
 * Dagama claims that the body of his king, Elizabello II, is truly a gift from the heavens due to his inhuman strength and fighting power.
 * In Episode 459 a nun can be seen praying "Dear Lord ... Where is this world headed?"