Question: If God is all-powerful, why can demons defy Him? Where do they get their power?
Response: I have consistently differentiated between the supernatural/miraculous that only God can do, and the natural realm in which all created beings, including Satan himself, are bound by natural laws imposed by God upon them and the entire physical/spiritual universe. What Satan is able to do seems supernatural to us because it violates the laws as we know them that govern our time/space/matter dimension. Yet Satan is subject to God’s laws governing the spirit dimension in which demons ordinarily operate.
I don’t know how demons can intrude into our dimension and move things around, but I have no doubt that they can do so. Nor do I doubt that they do this on the basis of powers natural for their dimension of existence but which to us seem miraculous, just as gunpowder would seem miraculous to a native who had never seen it.
Though the power of demons is natural, it does not involve sleight-of-hand tricks that can be duplicated by stage magicians. I think that Jannes and Jambres opposed Moses (2 Timothy:3:8) by the power of Satan, not by stage magic. Nor do I think it was by legerdemain that Satan transported Christ to the top of a mountain or to the pinnacle of the temple. It is true that I have not witnessed these things for myself, but I have had enough eyewitnesses tell me of fire dancers (not the kind we have here who take four steps and are protected by the laws of physics) who get out in the middle of a huge fire ring in Malaysia or Fiji and dance around for many minutes, after having fasted and prepared themselves for many days for this feat. I don’t think any stage magician can duplicate it, and I am convinced that they are using a kind of “power” that we in the physical realm on Earth generally know nothing about.
But even so, it’s not a matter of “defying” God at all. Everything in our universe is created by Him, and that includes all natural forces, foreign to our senses or not.