A great 2nd edition resource is the Faiths & Avatars book, which gives a really good write-up on the gods and the beliefs of their followers but the truth is, The Forgotten Realms, as presented, is a somewhat unrealistic approach to ancient polytheistic belief systems. So, drawing upon what I know from Eliade's The Sacred and the Profane, Burkert's Greek Religion, Yehezkel Kaufmann's History of the Religion of Israel, and a couple of other sources on pre-modern polytheistic belief structures. I would love to have gotten my hands on M.A.R. Barker's article "Create a Religion in your Spare Time for Fun and Profit." (I hear they have a copy at DriveThruRpg.) This blog entry over at Game Over provides a nice review and summary of the article for those interested and it has helped me quite a bit.
To build on this, we honestly need to isolate a number of cultural/linguistic groups that inhabit the Realms and determine how tradition and ritual (which are very, very important parts of religion) interact with the societies. There is one, singular pantheon for all of Faerûn and that is something that really just pulls me right out of that precious suspension of disbelief because of how dramatically unrealistic it is. After doing some thinking, I managed to pull together some ideas about how cultural interaction and communication led to a diffusion of deities throughout the Faerûnian ecumene. This would be similar to how, during the Hellenistic and Roman periods of Mediterranean history you start seeing gods being identified with one another, mixed, and new gods form, like Serapis in Graeco-Roman Egypt, Zeus' syncretistic identification with other chief deities, and the introduction of entirely foreign cults, like Mithraism from Iran.
So, I started working backwards through Faerûnian history to break down the human deities into distinct and isolated pantheons of gods. These are based on language groups that are also cultural groups of humans that dispersed through Faerûn--Talfiric, Calishite, Luskan, Damaran, Netherese, and Chondathan. The Netherese gods are detailed in the 2nd edition boxed set entitled Netheril: Empire of Magic, although it is important to know that Chauntea (whom I identify as a Chondathan grain goddess) was synonymous with the Netherese goddess Jannath.
From there I started to do some work mixing and matching, looking at which gods were worshiped the most where through the sourcebooks and discovered, low and behold, that I could pull out a logical sense of consistency for a lot of these different pantheons. It took some work but hey, back then I was a lazy college student who liked wasting time applying my new scholarship skills to frivolous stuff like D&D back in the day.
So, what are the values of these cultures? Well, for starters, I'm going to take a look at the Chondathan culture before I start tackling other ones. Chondathan culture is a very distinctly Western European plus good ol' corn-fed United States red-blooded American sturdiness. The Chondathan diaspora has spread out from it's titular lands, through the Vilhon Reach, Cormyr, Sembia, the Dales, and out into the Western Heartlands where they mixed with and culturally replaced the Talfiric peoples. Basically, the population of the Heartlands of Faerûn, from Waterdeep to the western Sea of Fallen Stars littoral, is almost entirely of Chondathan linguistic speakers. Language and culture are intrinsically tied, so much so as to be difficult to separate one from the other, therefore, even if the genetic stock of a region isn't Chondathan, if they speak a Chondathan language, it is safe to assume they have overall Chondathan cultural traits.
Anyway, I'm digressing. A lot. So, let me circle away from this tangent and back to my main points. Alright? Okay.
Now, while reading the review of Barker's essay at Game Over, the blogger emphasizes one of Barker's key arguments about how to build these religions in his summary:
It's not just a matter of phoning it in by whipping up a few names and one-paragraph descriptors for your gods either; these are entities which are going to govern the lives and livelihoods of player characters, and so you're going to want to answer a lot of questions about the gods and the religions from the players. ...
That said, the single-paragraph description of the god is still there; Barker demonstrates just how much of a religions' nature can be derived from examining a detailed and well-written description of its god. However, it's not actually Barker's recommended starting point. ...
... I bring this up partly to illustrate that the essay has considerable value beyond the issue of building religions in fantasy games, and partly to demonstrate how Barker actually recommends building religions: from the ground up. Worry about the social, the political, the economic situation on the ground, in the cultures where the player characters move - then get your arse up there and think about what kind of gods might appeal to those cultures, or might have formed those cultures through exerting influence on their situations. Treat religion as the social process it is and you'll get something rich and deep for your trouble.
Start at the bottom then work your way up. The problem with most DMs, setting designers, etc., is that they never, ever, ever consider the actual duties and functions of their deities within the actual structure of the society itself. Oh, yeah, in the 2nd edition The Complete Priests' Handbook the writers and designers give all sorts of advice for creating gods, religions, clergies, etc. It's only the tip of the iceberg.
Where can one start?
With studying actual polytheistic faith systems that really exist in the world. No, I'm not talking about Wicca, Witchcraft, or Paganism in the modern world. Sorry, they're honestly new inventions and while they seek to reclaim and restore much of the lost religions of the old gods of Europe, they are still very influenced by modern perceptions of those cults than the actual worship and practice of ancient religious belief.
But permit me to digress, once more, by citing an excerpt of the 3rd edition Forgotten Realms Campaign Setting book.
Some Faerûnians zealously follow one deity. Others make sacrifices to many deities, while upholding one as their personal patron. Still others sacrifice to as many deities as possible, shifting allegiances as their circumstances and needs warrant. It's a rare Faerûnian who hasn't occasionally hoped to avert the baleful influence of an evil deity with a propitious gift, or thanked a good power for an unexpected blessing. The belief system of most Faerûnians generally centers on a particular deity whose interests and influences are most likely to affect them, but acknowledges other gods as significant and important, too. --The Forgotten Realms Campaign Setting, 3rd edition, pg. 93.The first sentence in this paragraph describes henotheism. The second sentence as well as the last sentence describe monolatry. Everything in the middle describes standard polytheism or (at most) kathenotheism. In function, however, especially through the cleric and paladin character class, what we mostly get in Faerûn is henotheism and monolatrism. This smacks me as problematic, especially because the common needs of the populace will require frequent appeals to a variety of deities.
If you read pages 93 and 94 of the 3rd edition Campaign Setting, it paints a fairly believable and solid picture of religion in theory but in practice most DMs simply take the easy way out--monolatry and henotheism, all clergy are paladins, monks, and clerics, etc. The Forgotten Realms Campaign Setting makes it clear that "most of a Temple's clergy are not clerics." It goes on to say, "They're experts, aristocrats, even commoners who serve as advisors and counselors to the faithful and officiate at routine observances. A cleric usually leads any particular temple, shrine, or order, judiciously using her spells to aid sick or injured followers and assist the local authorities in maintaining law and order in the community as it suits the deity in question."
Here, the authors actually undo some of what they were trying to build--true actual polytheism. The key here is where they say, "the faithful" and "followers." This can mislead players and DMs alike into the assumption that the clergy of a temple only administers to those who follow their deity and their deity alone.
When studying polytheistic belief systems, I tend to lean toward the Graeco-Roman traditions (specifically because I specialized in Graeco-Roman history, society, and culture in my undergrad and grad school days) and modern-day polytheistic traditions like Hinduism and Shinto.
I have a lot of exposure to Shinto through my stuc., and receive a good-luck charm that would bless their efforts in study with hopes that they would pass their university entrance examinations. These students were not henotheists or monolatrists like the bulk of D&D non-player-characters and player-characters. The temple provided a specific service to the students by taking offerings dedicated to Tenjin and interceding with the kami in order to grant his favor to the students.
So what do temples do? They provide services to the populace but also to the state. Their job is to be a basis for tradition so as to curry favor with the gods so that peace and prosperity is ensured, misfortune and catastrophe are prevented or warded off, and the cultural traditions continue. The D&D campaign worlds are far too influenced by Christianity (whether players and DMs like it or not) in that each god has a sort of salvation or promised afterlife for their worshipers. Relationship with the divine is a part of Judeo-Christian faith (more Christianity than modern Judaism, though). Polytheists don't often seek to have a personal relationship with the divine unless they're rulers or dies of Japanese history and culture, as well as having visited a couple Shinto temples in Japan. What I saw, for example, at the Tenman-gu shrine to Tenjin, the Shinto god of scholarship, made me realize that D&D (in particular) and role-playing games' designers (in general) have religion all wrong.
I arrived at Tenman-gu around the time when Exam Hell was just starting to ramp up. Students would come to the temple and make offerings to Tenjin in money, incense, etoracles and even then it is rare because too much attention from the gods invites calamity as much as fortune.
So temples don't exist for the individual to develop a relationship with the god. They're not where the populace go in order to commune with the deity. They're where the populace go to offer sacrifice and store-houses or treasuries for various offerings. They could also be a place where certain things are stored safely, like how the Temple of Saturn in ancient Republican Rome was the site of the Roman state's treasury and also where legal documents (like contracts, wills & testaments, etc.), archives, seals, and original measurement templates were kept. They could be clinics, like the temples of Asclepius, the Graeco-Roman god of healing, or places where sacred prostitutes were sought, like the Temples of Ishtar or Ianna in ancient Mesopotamia.
Alright, what does the clergy do? The clergy has knowledge. This knowledge could take many forms. It could simply be the knowledge of when and where to plow, when to sow and when to harvest. It could be analysis of the calendar in order to decide what days are fasti and which are nefas. It could be knowledge of medicine and healing, or how to examine entrails or flights of birds to divine the gods' will. Some clergies may have knowledge of exorcism and warding against ghosts, evil spirits, or plain bad luck. Still others will observe and maintain the markers of boundaries. Even irrigation and water management has been known to fall under the auspices of temples in Bali. Most importantly, however, the clergy of a temple has knowledge of how to carry out tradition, how to avoid ritual impurity, how to be cleansed of ritual impurity, when and how to store and display sacred relics and cult statues, when and how to perform ritual dances, how to offer sacrifice, what hymns to sing and their words, what songs or stories to tell and when, how to recreate and commemorate sacred events, and how to communicate to the gods both the fidelity of the populace and their needs, wishes and desires.
Notice that I say "the populace" and not "the people." Polytheistic deities were often perceived as detached and distant. This is especially apparent in Republican Rome where the primary gods were gods of the state. The gods were more concerned with Rome as a whole than with each and every individual living in the city or countryside.
A major important concept to note is that religion is going to vary from region to region, especially in the form of traditions, cultic practices, sacrifices, festivals, and other details. While in a wide Faerûnian ecumene it is likely that specific similarities in practices and traditions are going to be common, there is not much likelihood of there being any strict set of doctrines and dogmas. Shinto doesn't have much in the way of guiding texts or dogmatic scriptures, for example. Hinduism has numerous holy texts and different sects of Hinduism value the various texts differently. Buddhism has a great many holy texts. Greek, Roman, and Egyptian religion didn't have much in the way of holy texts that were monopolized by the temples and considered specifically holy and sacred.
Thus, a great starting point for actually building a believable religious tradition for your role-playing world will start at the ground and work its way up. A phenomenal example of a compelling religious tradition can be found in the video game The Elder Scrolls III: Morrowind. The Tribunal Temple, the primary religion of the dark elves of the setting, is full of lore, hierarchies, functions, dogmas, traditions, rituals, and volume upon volume upon volume of in-game sacred texts, scriptures, and writings that the player can read and participate in.