i have found this link:
In the Postclassic Period codices, this god used to be called (and in some books still is) god D. Worshiped at least as far back as the P...
ancientmayalife.blogspot.com
maybe the meaning of d in linked to the mayan God D :P:P
This is interesting because I expect something mesoamerican to be of importance. Basically I think Skypea is the meta arc, Oda shoved all the mythologies that he knows into this weirdly disconnected arc, and certainly not without reason.
Which also leads me to my main theory what D. is.
I think it likely means Dingir. Dingir (pl. Dingirdingir) is the Sumerian symbol for a god. Alone the symbol means "An", the main sky father deity. But as a prefix it is read as a silent initial d, for example dIM would literally translate as "Storm God" but means Baal Hadad (also known as Adad or Iskur), the canaanite storm deity who has the same symbols as Luffy, the Bull (Luffy's Zodiac and also Ucy in the Corrida Colliseum) and the Thunderbolt (well Luffy is immune to Thunder but that is still an association) and while with these symbols he clearly mirrors Zeus (who is violent and gluttonous like Luffy or his son Heracles, the role model for Oden) the Greeks actually equated Hadad with Kronos (rom. Saturn), since he was also a fertility deity (as storm and fertility is the same in dry regions).
And Saturn is associated with the Devil in some weird esoteric and conspiracy theories and is visually represented by the Straw Hat.
But not just the Saturn, Baal Hadad is also presented as an evil figure in the old testament as a result of monotheism not really liking polytheism.
And there are more of these Dingir, who commonly appeared as horned winged humans (Bilkans, huh), that have this association as devilish figures.
Now you might say "That's super fringe knowledge, how could Oda possibly know this"... But it is already inside One Piece!
Enel is named after Enlil, a Sumerian Storm deity that at one point replaced An as the main God. Enlil is known as the separator of heaven and earth, which is exactly what Enel tried to do. The holy number of Enlil is 50, and Enel had exactly 50 soldiers in his army.
On top of that: The Dingirdingir, here better known as Annunaki are the gods that are associated with weird ancient alien astronaut conspiracy theories. Someone called Zachariah Sitchin once wrote a book that became hugely popular, where he claimed the Annunaki are actually space traveling aliens from a planet called Nibiru.
And what did Enel do? Apparently he traveled to the homeland of his ancestors on the moon.
The references behind Enel are so spot on, that I can indeed assume that Oda is familiar with the ancient Middle Eastern religions. Especially because the Sumerians are also credited as the inventors of cuneiform writing, which closely resembles the Poneglyphs. This form of writing largely disappeared with the Bronze Age collapse, which roughly happened 1200 BC, the "negative value of 800 years ago" (Void Century).