Who is the god(dess) of the temple? I've had different approaches to this over the years. I've invented religions, I've adopted religions from history, I've used religions from literature, and I've used religions from the D&D universe (specifically, the 'Nonhuman Mythos' from the old Deities and Demigods - the one that claims to have the Melnibonean and Cthulhu Mythos in, but doesn't. There weren't really any other 'D&D' deities back then, and even now alongside Pelor and Fharlanghn or whatever, they've used Loviatar and St Cuthbert - taking real myths and projecting them into the D&D world, though in St Cuthbert's case, any resemblance to the historical St Cuthbert is minimal). I've adopted religions from history and changed the names so that they don't seem to be the cults that they are. I've also assigned generic names to deities ('The Sky God', 'The Moon Goddess' etc) to convey a flavour without specifics.
I still don't know what the best way to play this is. Do the players really care about the cults I've invented? Who but me cares if my Sky-God is called Vondar, if he's just the same as Zeus? Same with using Manwë or Mörnir. What if my players aren't as nerdy as I am? What if they don't know who Mörnir is? What if they don't even know who Manwë is? Isn't it better that they just know that the Sky God is a sky-god? If that's the case, why not call him Zeus? The players have some idea who Zeus is, and their characters would know something about the Sky God, even if they're not followers of his cult.
But I'm not sure. It sort of seems like cheating somehow, to have Loki and Isis and Nuada as part of the mythology of the current campaign, without at least changing the names (currently, they exist with more-or-less subtle pseudonyms, mostly anagrammatic). I'm not setting the campaign in 5th century BC Egypt, or 2nd century AD Ireland, or 10th century AD Norway. So why the gods from those times and places?
The monster lists include mythological monsters (like Sphinxes), monsters from literature (like Orcs and Hobbits, sorry I mean 'Halflings'), and made-up monsters (like Owlbears). So maybe, the lists of gods should include gods from mythology, gods from literature, and made-up gods.
In fact, this is how my religions currently work. There are many gods from real-world religions - Egyptian, Greek, Babylonian, 'Central American', Japanese... culled from DDG. They're not quite as in our own history however, as I've not used entire pantheons. I've randomly determined gods from the pantheons, and sometimes combined pantheons - so what are the religions of two cultures in Earth history are combined into the multi-layered myths of single cultures. But then, Gygax had 'Gorgons' and 'Medusae' as two different monsters, while there are no Satyrs in Basic - that's kinda the same thing isn't it?
I also use one particular set of fictional deities. The reprints of DDG still included the Nehwon Mythos, so I use them. Why not? No-one in a polytheistic society would know all the gods that they might come across, especially if they travelled around. I can't imagine that someone from Lydia, for example, if they travelled to the Rhineland in the 3rd century AD, would know who all the gods were, though they would surely know many of them. If they went to Persepolis, they might have very little idea at all. So I don't mind some ignorance on the part of my players (who don't know the Lankhmar books at all).
I also use some made-up D&D 'official' gods. The nonhuman deities are the basis for the pantheons of my non-humans, though (in line with the charts in the back of DDG) some of the human-culture deities can have non-human clerics.
So is that the right balance? I still don't know. I don't think it is. I'm considering dropping the names of my disguised 'real' gods. Isis, Bast, Horus and Set or Isi, Tas, Rosh and Ets? Or, even the Mother Goddess, the Cat Goddess, the God of Vengeance and the Snake God? It's likely to become a situation that I need to resolve in the next few days - the party could do with some healing, and anyway, they've killed some Snake-Cultists. The Mother Goddess, the Cat Goddess, and the God of Vengeance all really hate the Snake God and would probably be happy to help out a party that had broken up a secret Snake-Cult cabal.
So where is the optimum split/point on the spectrum or even answer? I'm thinking I may as well just call Isis Isis, if she's Isis. It may just be easier. At the same time, I think I'm going to include more literary pantheons - I don't actually have either Manwë or Mörnir in my current campaign world, maybe I should have some Elves who venerate the Valar, some Celto-Nordic types who venerate Mörnir and Galadan, barbarians who follow Crom as well as both Kos and Tyr (or, 'Yrt').
That might be fun...