I started Yoga last week. After nine years of jumping out of airplanes with heavy rucksacks, my body is pretty beat up and high-impact exercise does more damage than good. For the first time since I broke my hand two years ago, I was able to get a great workout five days in a row without considerable pain the next day. The place I go has a strong foundation in the spiritual side of things, which also led me to doing some reading on Buddhism (Okay, I bought The Complete Idiot's Guide to Buddhism, but it's a start.) This, in turn, led me to start thinking about spirituality in gaming.
Religious practices in games are in many occasions just a blank filled in on a character sheet or merely a source of power for special abilities. In a world where divine majesty is routinely manifest and quite real (compared to ours where divine inspiration is impossible to prove empirically), I would imagine that religious practices would be much more serious than they are now, or even how they were historically. The fact that a local cleric can heal an injury as easily as Tony Hawk can kickflip brings the power of the divine to the forefront in a way that any real-world clergy would never be able to. By my estimation, even epic events, such as Raise Dead or Power Word, Kill spells happen with greater frequency in fantasy worlds than the miracles of our real world's faiths.
To me, this means that there would be no atheists or agnostics in Fantasy RPGs. It's hard to not believe in gods when it's as much a part of life as the sun rising in the morning and setting in the evening (Unless you are playing Lamentations of the Flame Princess, which always occurs close to the Midnight Sun effect in my mind). What I would see is a much more pronounced set of beliefs and actions which keep the gods' favor in mind, similar to the way polytheistic cultures (or monotheistic cultures who revere saints) operate.
What does this mean for games? I guess it's one of those things that the individual group wants to introduce. Each god would likely have a set of strictures that devout followers would need to follow. I remember dreading eating dinner at my grandparents' house on Friday nights because I hate fish. I am sure that every diety has actions they like and dislike. In a busy cosmology like most Dungeons and Dragons campaign settings, these can stack up fast and would logically result in the following of pantheons and the strictures within that pantheon. In the Greyhawk setting, I could easily believe that citizens of Ekbir would pay homage to the entire Baklunish Pantheon, but would not be too concerned with that of the Elves (and vice versa).
The Mongoose edition of RuneQuest has some interesting insights onto religion in the fantasy milieu, with the God Learners actually manipulating divine essence to remove other deities from existence. Warhammer Fantasy Roleplay (in it's many varied incarnations) also takes an approach that many other games don't, elevating one god (Sigmar) in the primary campaign setting, but minimizing him elsewhere in the world.
If you want to take a realistic approach to religion in a game where magic and dragons exist (certainly pushing the boundaries of the term "realistic".), I'd make sure that there are those little flavor details. We're talking little details like fasts. It could be a dietary fast, something as great as not eating or drinking at all from sunrise to sunset during a holy month (a la Ramadan) to something that has minimal game effect (no red meat on Fridays). It could be something that takes up time, like a daily Yoga regimen followed by meditation (although, if you are a long-term Yoga practitioner, you had better not have low physical attributes).
I am challenging myself and all of my readers to make religion a larger part of their characters' lives. I would love to hear some of your thoughts and practices of PC religion.
A Side Thought On Atheists: God-Killers
The fact that there is incontrovertible evidence that there is a higher power doesn't mean that you have to be in faithful worship of one or many of the deities. In an epic campaign, you might have a character who wants to kill God (or the Gods), a la Preacher (A wonderfully Tarantino-esque comic by Garth Ennis and Steve Dillon filled with blood, vulgarity, and western imagery). Deicide is not a new concept in Fantasy RPGs. Look no further for the Forgotten Realms' Cyric for inspiration. I'm no expert, but as I recall, he killed three deities (Although Bane was just too cool to keep dead) and became a major player in the Realms Cosmology. Maybe your campaign could bring Atheism to the world by killing the gods themselves.