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 were introduced 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 400 years prior to the current storyline, and Gan Fall was the previous God 6 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 from Luffy. After Enel was gone, Gan Fall reclaimed the title.

"Gods" of Shandians
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 Norland and his crew arrived at Jaya and cured the tribe of Tree-Rush.

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
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 she had been referring to Gan Fall.

There's 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 gods 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 you pick one up, you may have the contents, though you'll need to replace the 'offerings' and put it 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 deat 'god' in Sky Island.

In Davy Back fights it seems customary to throw 3 coins into the water, as an 'offering' to Davy Jones.

Bartholomew Kuma has a bible (notable as it has "BIBLE" in full capitals) in his hands and always carries it with him. Gecko Moria 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 Arabasta looks to be a Middle Eastern land, their graves are cross shaped.

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

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

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.