Do Demons Have Some Domain Over the Physical World?

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How much power do demons have over the physical world? Some of my friends claim experiences with seeing ghosts, friends being inhabited, and even furniture moving. I've heard people advanced in the occult, when they touch a holy medal or some such, experiencing pain and a flash coming from the holy object. According to some exorcists, demons can even give a stigmata. Do you think those kinds of things can happen? Are there other things that can happen as well? If it's true, personally, I hear more about that kind of thing that about seeing or experiencing angels. Why would we experience demons but not angels as much? Why would God let demons have control over the physical world at all?

In the Bible, in the writings of saints, and in contemporary accounts like those mentioned by Countess and Farmer, angels come, fulfill a specific purpose, and go. They do not seem to be on earth in general. So those would be my only sources, the Bible, the writings of saints, and accounts from acquaintances.

Okay but those accounts say nothing like angels could be discovered by Google or videotaped so they don't come around as much anymore for fear of discovery. As you pointed out the Screwtape letters say demons hide themselves because that benefits them the most but are God and the angels playing by some kind of rules that don't favor them and so they have to allow Google to catch them?