My upcoming fantasy campaign features am atheist world empire. The High Elves found a simple version of the scientific method in their bronze age, and applied it to everything, including gods and magic, to find out that those didn't exist at all. A thousand years later, they're at Rennaissance level technology and rule the world.
Some background: About two hundred years ago, the High Elves were attacked for their blasphemy, fought back, got sick of all the superstition of the other peoples and subjugated them, forbidding all gods and claims of magic for the con jobs that they were, not neglecting to actually prove time and again that the gods were just fairy tales and magic just tricks or coincidence, because they did not act in the ways believers claimed they would. Ridicule was, of course, also used to make sure everybody gets the new party line. The political system is still a relatively simple feudal aristocracy, though, with the subjugated kingdoms having a loyal viceroy out of the local nobility, and the emperor doubles as the de-facto-king of the original high elven kingdom. There are two other elven vice-kingdoms, one orcish one (these orcs are of the short-lived rapid-breeder type), one with various human cultures (from Central Europe to Iran, with an African principiality thrown in), a China-inspired lizardpeople kingdom, a Japan-inspired dragonmen island, and the high elven colonies in the West with their Aztec/Native American/Indio-inspired cobra-head humanoid snake people. Dwarves are a minority from the original elven kingdoms without their own nobility. Today, after two hundred years, only the eldest elves and Dragonmen even remember the names of the old gods. Medical technology is relatively simple, of course, with no anorganic medicine. While medical care is widespread and affordable for everyone due to Imperial law, vaccinations and antibiotics are unknown, and there are various mean diseases, so only a third of children gets past the age of six. Elves and Dragonmen age slower than others, with 600 years being an theoretical maximum age that few reach due to disease, very occasional famines and, in earlier times before the Unification, wars. The other species have shorter lives of various length.
Over the past 200 years, migration across the world empire has spawned minorities of each people in each other vice-kingdom. The world has become a lot more cosmopolitic. Species cannot mix (that simply doesn't work biologically), so there are no half-breeds, but most people regularily interact with people from other species. Not everybody likes that, of course.
And then, at the start of my campaign, magic and gods actually come into the world. Not the old gods, who are mostly forgotten now and (I have decided) never actually existed anyway, but new ones, who actually do exist.
The System is GURPS, but the Magic is RoleMaster's magic system, with its threefold Channeling/Essence/Mentalism system. Basically all spell lists are allowed (though some magical items are impossible, especially anything that affects the amount of spells one can cast), but there's an Advantage requirement for access to various types of spells of the three magic realms, and you need to spend points for getting your Magic Points. Pretty expensive, so adaption in most cases should be slow, with a few exceptions for dramatic reasons (of which the players, who are encouraged to save a few points at the start, may well be some). However, once having access to the spells and the required amount of Magic Points, actually casting even high-level spells isn't all that hard.
What I would ask you, dear reader, to do is give me an idea how people, players and various NPC's of all types alike, will, in your opinion, react to the clear evidence of actual magic and gods in a world that has been sure for centuries that such things do not exist, and how the magic of Spell Law will affect the balance of the world. I hope this will help me anticipate at least some of the things that will come up in the campaign, and have answers ready when the need for them comes up.
