The first idea is that they're a cosmic counterbalance. Good and evil exist in balance and in order to keep that balance demons must exist.
The second idea is that there are beings that are simply unknowable to humans but are distinctly malevolent. Demon is just a label for these monsterous things.
Their mere existence mutes God's claim to omniscience, then. Not that this will stop Christians at all.
Well, it depends: what you want the demons to be? counterpart angeles? sins hunters? tempters?
that decide a lot how they are going to be.
"My Name is Bolt, Bolt Crank and I dont care if you believe or not"
![[up]](https://static.tvtropes.org/pmwiki/pub/smiles/arrow_up.png "[up]"){kind=icon}
Well...only if you take a very strawman view of Christianity.
I prefer them as the evil counterparts to angels.
Then I must ask....what are your agenl them?
Becuase this depend of what your angels are, you have three choices.
1) they are a diferent faction of angel.
2) they are fallen angel meaing any powers they got by corruption or tampering while the other are "pure"
3) They are manifiestation of evil in equal but diferent category of angels.
So if the angel protect, they destroy and so own.
"My Name is Bolt, Bolt Crank and I dont care if you believe or not"
I would go with option three. Not to mention their origins would be akin to the Islamic description of djinn: being born of smokeless fire.
edited 20th Apr '18 7:09:12 PM by superboy313
The demons are the Chaos of fire, opposing the angels that are the Order of...? Still water? Ice?
(See if you can work in some wind, weight and time... 
)
edited 17th Apr '18 11:18:19 PM by DeusDenuo
I'd go with still water.
In all seriousness, angels would be formed from light.
Water might be a good idea, being "the origin of life" and all that.
Ok, if your demons are counterpart, you need to clarified what your angels are all about and make a mirror version of them and presto, your own angels and demons.
"My Name is Bolt, Bolt Crank and I dont care if you believe or not"
怒炎止水, if you wanted to give it a four-character idiom.
Basically, angels are all about maintaining order and stability in the cosmos. While most are generally good, others take the fight against demons to fanatic levels.
Rather than being angels who rebelled against God, they were a byproduct of God's creation. When the universe was created, demons came into existence in the process. Think of them as an unintentional creation instead of being directly created like angels and humans.