Regarding the Steve Jackson Games
version of In Nomine:
The SJG version of IN is very different from the French game. The French game is an extreme piece of satire in regards to Catholcism, and the entire worldview is slanted in that direction - non-Christian faiths are given short shrift and non-Catholic flavors of Christianity are considered wrong on several points.
SJG's game, on the other hand, is respectful towards the ultimate subject matter, and attempts to forge a "hardly anyone is exactly right, but hardly anyone is exactly wrong" approach. At the same time, it is still mindful of its satirical origins and isn't afraid to have some completely silly bits - after all, you're playing angels and demons, how serious can it be? In the end, this makes it less offensive to the largely non-Catholic market in the United States and adds a lot of depth to the game.
As to the subject of the game itself:
Heaven and Hell are engaged in a cold war. God doesn't answer calls much nowadays, leaving Heaven led by the Seraphim Council, consisting of the Archangels and other old, wise angels. Hell, as always, is led by Lucifer, the Lightbringer of Christian mythological fame, with several Demon Princes below him.
The various Archangels and Demon Princes are also taken from Jewish, Christian, and Islamic legend, with each attuned to a Word - a concept in the universe (or Symphony as referred to in-game). For example, Michael is the Archangel of War, and Vapula is the Demon Prince of Technology.
Players play servitors of these Archangels or Demon Princes - this may entail playing a Celestial (angel\demon), or a Soldier - a human who has been attuned to the Symphony and can use Songs (IN's "spells") as a Celestial can. There's advantages either way - Celestials are powerful, but Soldiers belong on Earth and thus do not cause Disturbance - noise that those attuned to the Symphony can hear - when they act.
In general, the world of In Nomine is the real world. While celestials do influence things, the core ideal of In Nomine that separates it from similar games such as those set in White Wolf's World of Darkness is that humans matter. Celestials aren't behind every significant event, and while celestials can be quite powerful by human standards, they don't make them poo.
One sticking point of In Nomine is the manner it separates "divine" and "pagan" religions, and this may offend. Divine religions ultimately worship God in the form of a single driving force in the universe. Pagan religions actually worship Ethereal 'gods' created by humanity through their dreams. While IN tries to foster some uncertainty - the Ethereals insist God is really just an Ethereal who got powerful enough to rewrite the universe - this could be a sticking point for some. In Nomine also categorizes some Earthly religions in an odd fashion: some are clearly divine, such as the Abrahamic religions. Others are clearly pagan, such as the gods of Shinto. Others are considered an odd mix as they revere dieties who are ultimately expressions of a single force - Voudon and Hinduism are the examples used in In Nomine's canon. While this really shouldn't be a surprise considering the game's origins, this point has offended people on both sides of the issue. (Christians offended by the liberties the game takes with their beliefs, or Neo-pagans who... are offended by the liberties the game takes with their beliefs. You can't win.)