The idea of a war between angels and demons playing out on 21st century city streets is a popular theme for video games and movies today. Legion, Constantine, The Prophecy, etc. Other stories focus on Satan in particular, and that can be kind of cool in its own right. End of Days is no Devil's Advocate but Gabriel Byrne is still pretty incredible in the role of Lucifer.
There are a handful of ways to go about incorporating something like this into games. d20 Modern is a very modular system that can be tweaked to run just about any kind of game. It even has a sourcebook about different ways for the world to end and I think the Biblical story of Revelation is one of the options presented in the book. In Nomine is far more obscure but it's a stand-alone game with precisely this in mind: the players are running either a group of low-ranking angels or fledgling demons and must carry out their shadow war under the noses of mortals, all while trying to advance in the infernal or celestial hierarchies. For a brief time there was a game called Nexus War, modeled after Urban Dead,where players assumed the roles of angels, demons or mortals stuck in between battling in a kind of Purgatory that had been plucked from time and space. Everyone stuck there was supposed to battle it out to determine the fate of the next world.
Right now I've been reading a lot on Demon: The Fallen, set in White Wolf's "World of Darkness" setting. It was published in 2002 so it coincides with that period's "Final Nights" timeline for the various games in that setting. What sets Demon apart is that there aren't actually any angels in the World of Darkness at all. The Age of Wrath concluded with the demons being cast into Hell. But the walls of their prison was sundered in a great storm in the underworld, and cracks formed large enough for the weakest among them to slip out and return to Earth. When they arrived, they saw that the world had largely been abandoned, which helps to explain why there are so many wizards, vampires, werewolves, ghosts, faeries and other unsavory types running around.
Has anyone ever played any of these games or run a game like this before?