D&D General building a faith around the assumptions of the cleric instead of in spite of it?

[Mind of tempest]

Since you are looking for possible models on a polytheistic temple these are real world ones that a bunch is known about.

Hinduism is a diverse living polytheistic faith with over a billion followers so it is a possible model (and from the diversity, multiple possible models) to gain an insight on how to do a polytheistic faith in a game. Wikipedia articles, children's books about Hinduism for Hindus, The Idiot's Guide to Hinduism, fairly concise online articles, library copies of the Teaching Company's college classes on introduction to Hinduism (audio CDs or DVDs) are all possible pathways to getting an insight.

Different strains of Buddhism and Taoism include polytheistic faiths as well and could be similarly used as different models.

Ancient classical and to a lesser extent Egyptian polytheism has a lot of research and knowledge about them as well and similar researches could be done. For western fantasy audiences this is typically the mythology base that comes to mind first and is most familiar, even if the actual religious and temple and worship aspects are not familiar to most modern Western audiences.

These are if you want a realistic model for your game's polytheism.

This is for a game so other priorities are reasonable too.

Other models could be media genre ones, if you like pulp fantasy genre stuff then reading R.E. Howard's Conan stories for how various religions and religious practices are portrayed in different stories would be a great basis, keeping in mind cultural sensitivity issues as you read 30's pulp fantasy. He is a great writer and different religions and religious practices come up in stories involving Stygian Set worship, Pictish religious practices, Mithraism, and Conan's relationship with Crom and Bel and such.

it is more organisational structure that I am lack rituals and holidays are easy to knit from whole cloth.

 

[Tonguez]

If there are two competing religions, then the simplest explanation is that one is a "newcomer", and there hasn't been much syncretization yet (this tends to eventually happen once the faiths begin to accommodate each other). A recent conquest, colonisation, or migration of a different people, perhaps. The dnd monks could even be the outcome of the first element of syncretisation, alternatively one of the religions might have a monastic element that is becoming prominent,

Rome is interesting because of the split between the traditional worship of domestic spirits (Penates, Lares) and the State adoption of the Greek pantheon.

So you had most roman families giving libations at household shrines or various neighbourhood shrines, sacred springs and groves - which lead to the various stories of nymphs and heroes (founding ancestors).

At the state level though you have the big processions and temples of the Royal Gods of Olympus - the Temples were not only about honouring the gods but also for promoting the ‘lawful‘ power of the state.
Which in a multi-ethnic Empire like Rome meant that many foreign gods would eventually be absorbed (the Romans also took on the Egyptian pantheon) and worship might come in different combinations (Rome had a lot of triune-gods).

priests, augurs, healers etc spread themselves across various levels of the structure from peasant shrines to Imperial temples