As some of you have realized by now, there's a good mix of Christianity and Hellenic mythology involved in the true nature of the world of my book. There's also a tad bit of Norse mythology when Freyja shows up as a character.
There have been five ages of man. First the Golden Age, filled with beautiful, wise, long-living people, who when they died became daemons, or beneficial guardians of the world. Second the Silver Age, where the people spent a hundred years as children, pretty much ignoring the gods and doing whatever made them feel good in the moment. They spent a few years all grown up, then died, becoming the blessed of the underworld.
Here's where I start to add things. These impetuous people were so bent on being childish and pulling pranks that they broke out of the underworld and started messing with the world for kicks, becoming what we know as fairies.
Third came the Bronze Age, where the people were constantly at war. After Judgement they were tossed into Gehenna (Christianity-inspired, obviously), but they were so good at fighting they didn't sit around and get tortured, they became the torturers (demons). As this Age ended, the gold daemons got really mad that each new group of peoples on earth was worse than the last, and in their rage they transformed into demons as well. They let loose the bronze demons, right as a new age dawned. Fourth the Heroic Age, where men were noble and generally good. When they died, they became demigods, or angels, whichever you prefer to call them.
Last is the Iron Age, the one currently going on. It's filled with misery and bickering, and people lie to come out ahead. The Greek gods have forsaken the world.
God, Judgement, heaven, and hell, are all truths in my world. Hell has three levels (see Worldbuilding: Geography and Climate), and heaven is largely unmentioned. Judgement began shortly after when, in the Bible, it was said it would happen soon, but since the population of the world is going up so quickly, the line in the place of the dead is only getting longer. Someday perhaps humanity will break the world, or it will fall apart from age, and everyone who remains will go in the line. Then God can catch up with everyone and Judgement Day (or Eon as the word also translates from Greek) will finally be over.
Magi have kept themselves secret from normal society, only ever knowing a handful of other magi in the course of their lives. There are several specializations they can take, such as necromancer, evoker, enchanter, diviner, etc. Necromancers and enchanters are almost always evil. Necromancers use souls of the dead to reanimate corpses, stealing them out of line for Judgement to do so. Enchanters tend to go into politics and use their persuasive magic to benefit themselves. Evokers used to be the most common type of magi, being close to the elements and expert at hurling them for massive bursts of damage, but in the modern world it's an impractical skill.
The magic system basically follows a d20 system, but I take a few liberties here and there. If I wanted to follow the rules exactly I'd petition for it to be published under the label of either Dungeons and Dragons or Pathfinder, but that's not what I'm going for.