The Bible clearly says they are not real. The Bible also clearly says that they are real.
I am a pragmatist. Most people examining deep philosophical and the theological questions with no clear answer are ignorant idiots.
Digression on quantum theory and too clever by half idiots:
Some of them, including perhaps myself, are not simple idiots but too clever by half idiots.
I will give those investigating the issue of the Bohr Copenhagen Interpretation of quantum mechanics, and the Everett Many-Worlds Interpretation of quantum mechanics credit for being very smart and very well informed idiots, but the rest of them are for the most part just plain idiots. Me, I favor the Minecraft Interpretation of quantum mechanics. Reality is video game generated by God. He instantiates the people he chooses, and instantiates the the things that they observe, including the people and creatures that they observe, but does not necessarily instantiate the people they observe quite as fully, and similarly down the line for the things observed by people observed by people observed by people chosen by God for full instantiation. So the far side of the moon is real, but its reality runs considerably thinner. Merely discussing this speculation makes me yet another too clever by half idiot also. But I have an excuse for trying to be too clever by half, for if you try to apply either the Bohr interpretation or the Everett interpretation in the case where there is more than one observer, you get into trouble, while both solutions work if there is one supreme observer who gets to make things real, and every other creature gets to make things real to the extent that the one supreme observer makes that creature real. Of course, it is quite possible that the only reason we get into trouble is that we are not smart enough. Or, as Roger Penrose speculated, that our theory of Quantum Mechanics is wrong, and inherently fails physical tests in some cases — complex biological structures, quantum systems of very large dimension, and gravity. But the God theory is much easier to comprehend than Penrose’s somewhat incoherent speculations in the direction of a better theory, so I am going to stick with it, while keeping an open mind in the Penrose direction. And few people are smart enough to notice that both Bohr and Everett become incoherent with multiple observers of equal status, so they should stick with Bohr or Everett. Indeed few people are smart enough to comprehend what Bohr and Everett were speculating about, so the rest should stay clear of quantum theory altogether.
Back on topic : Demons
There is an excellent practical reason for disbelieving in demons. If you do not believe in a demon, then this radically reduces its power over you.
Thus, for example, if you believe that the Awesome and Mighty Covid Demon is merely yet another new flu, then when you get Covid, all that will happen is that you get another flu. And if you also believe that if you have never been jabbed, and came down with Covid, you are now a Golden Pureblood and will never get Covid again (though you well might get yet another new flu) then you will never get Covid again.
Similarly, if you do not believe in the Depression Demon, you are not going to get depression. You might well get sad when bad things happen, and sad and fearful when it seems like bad things might well happen, but you are not going to get depressed. If bad things do happen, you will get over them, and if they might well happen but they don’t you will get over that even faster.
Similarly, if you do not believe in the post traumatic stress demon, you will not suffer post traumatic stress. You will of course get mighty stressed when something traumatic happens, but then you are soon going to get a good nights sleep and hearty meal, and laugh about the bullet missing, or the bullet hitting, as the case may be. You might occasionally wake with a rapidly fading vague memory of a nightmare, which you are unable to remember clearly on waking, and completely unable to remember at all a few minutes after waking.
Depression and post traumatic stress are iatrogenic diseases. They are a demonic priesthood putting a hex on their faithful.
There is also an excellent practical reason for believing in demons. It makes the behavior of our ruling elite so much more intelligible, and thus enables you to put on the armor of God, protecting you from being driven crazy by their craziness.
As Charlie Kirk, martyr for Christ, told us, the proposition that Church and Easter are inessential activities, but bars, race rioting, and burning down Wendy’s are essential activities makes so much more sense if you assume our rulers are possessed by demons. Really, it is hard to explain Fauci weaponising a formerly harmless bat virus any other way, and even harder to explain why he has not yet been executed for crimes against humanity.
Charlie Kirk’s assassin was radicalised by hanging out on forums full of trannies, and a whole lot of trannies give an overwhelming impression of being dead bodies controlled by hostile alien entities. When one runs into a trannie, it is sometimes hard to disbelieve in demons.