Angels and demons, collectively termed celestials, are sorted into different Choirs (angelic) or Bands (demonic). Musical themes permeate the literature. For example, all of creation is collectively termed "The Symphony," and "spells" do not exist; celestial powers are either resonances, attunements, or Songs. Every Choir or Band has a "resonance" associated with it, a unique way in which they interact with the Symphony that functions as a supernatural ability.
A celestial generally also has two sets of "dissonance conditions," that determine how they acquire notes of dissonance. One set comes from the base Choir or Band, and the other set comes from the Word they serve. Violating these conditions involves fundamentally rejecting their identity (for demons) or their place in the Symphony (for angels), and this rejection manifests itself in dissonance. Dissonance impairs a celestial's abilities in certain ways, and is keyed to spiritual anguish. For example, Seraphim see lies plainly (via use of their resonance) and in turn cannot bring themselves to lie without betraying their natures (which earns them dissonance). Acquiring notes of dissonance, in turn, interferes with their ability to use their resonance. Thus, once an angel starts distancing itself from the Symphony, it runs the risk of getting caught in a vicious cycle that leads it further and further from its angelic nature - and down the path to Falling. Bands do not tend to have a similar problem; redeem a demon typically requires Superior intervention, whereas an angel can Fall spontaneously by itself.
Angels and demons usually work for one of the thirteen main Archangels or fourteen main Demon Princes. These are celestials who are each bound to a powerful "Word," or fundamental concept. Not every Word-bound celestial is a Superior; only the most important, most powerful, and most knowledgeable earn Superior status. For instance, Michael is the Archangel of War. Doxas, Angel of Glory, is his lieutenant and, while respected, does not bear the title and responsibilities of Archangel. Generally angels must ask their Superior to sponsor them for a Word before the Seraphim Counsel. Once they have asked their superior if they may be granted a word, they will have to explain their reasoning for being gifted this honour. If their answer is to their superior's satisfaction then they will be brought before the Seraphim council where they will be given a task to perform. (Usually in some way related to the word they seek to obtain). If there are any other angels who wish to also have this word, they would be tested simultaneously. The one who successfully completes the task to the best of his/her ability would then be granted the word.
Demons have a similar task, rallying support for becoming Word-bound. But because their is no demonic equivalent of the Seraphim council, they have to either request their Superior intercede for them, or go into Lower Hell and speak with Lucifer himself on the matter. Depending of his mood he might kill them on the spot, grant them a Word on the spot or give them a task to fulfill. But completing a task does not ensure the word is theirs, he may deny them the word, grant them a lesser word (see. Furfur Demon prince of Hardcore), or grant them a word they didn't want (such as stale bong water).
The Archangel or Demon Prince which a character works for shapes their nature, personality, abilities, and restrictions as much as their Choir or Band. Words can change subtly if a Demon Prince redeems or an Archangel Falls - for instance, Andrealphus, the Demon Prince of (selfish) Lust, was once the Archangel of (selfless) Love. Both Heaven and Hell (especially the latter) are divided on many important matters. For example, Michael's militant stance towards Hell clashes with that of Novalis, the peace-loving Archangel of Flowers, and that of Jean, Archangel of Lightning, who favors a moderate approach between the two extremes. Most Archangels are hesitant to support one particular religion, but Dominic (Judgment) and Laurence (the Sword) take extra pains to promote Catholicism, while Khalid (Faith) is avowedly Muslim.
The game offers no definitive answer as to whether the fallen angels had justification to rebel, although in practice, most games tend to default to the assumption that Heaven is in fact seeking to further the overall cause of good, and Hell to undermine it. One can play a "backwards" game as freedom fighters based in Hell fighting against angelic oppression just as easily as a "straight" game where sympathetic to the angels. To accommodate both these perspectives, game literature often describes a morally gray universe, where angelic and demonic forces sometimes work together for mutual benefit. Both Heaven and Hell are home to a great many political schemers and powerful secret police.
Though there are numerous powers players can purchase with experience or earn through missions, the rules themselves are at a level of simplicity that stands in stark contrast to the complex politics and baroque cosmology. Most agree that this results in In Nomine achieving flexibility and simplicity, but not game balance.
The character point values for Celestial characters in GURPS In Nomine are very high in relation to most other GURPS settings. Starting characters can easily exceed 800 points, and more experienced characters are likely to exceed 1500 points.
In Nomine was, with Steve Jackson himself giving permission, briefly an online interactive fiction role-playing game within the World Weavers role-playing game group. The "game" was much more story-oriented, with the dice rolling and GM role phased out in favor of players/writers coming up with their own twists and turns in the game. Though there was initial interest in the concept, there were too few players to sustain an active In Nomine component in World Weavers, and the game was shut down after several months.