I can see this subject matter taking us around the world and back.
Lets get one thing said out in the open. Do we agree that some spirits and some angels that are written about in the bible are equal to demons?
Now lets think about Job (the book of Job in the bible)...he was well thought of by God but satan said God had put a hedge around him and kept him protected. Does it say anywhere (that anyone knows of) in the bible, why God had a hedge around him? Did Job do anything wrong in God's eyes to deserve to take the hedge away and let satan have his way with his life? What exactly was that hedge?
Did satan put God to the test? Did God take down that hedge to prove that Job wished to honor God even when he was afflicted in so many ways. Isn't satan still the adversary who wishes to testify against us to God? Will not God take some hedges away to prove His point?
Jesus Heals a Man Born Blind
John 9 As he went along, he saw a man blind from birth. 2 His disciples asked him, “Rabbi, who sinned, this man or his parents, that he was born blind?”
3 “Neither this man nor his parents sinned,” said Jesus, “but this happened so that the works of God might be displayed in him.
Will not some sickness be caused by our own sins or even the sins of our fathers, just due to the corruptness of what we have done to God's creation.
Exodus 20:5 Thou shalt not bow down thyself to them, nor serve them: for I the Lord thy God am a jealous God, visiting the iniquity of the fathers upon the children unto the third and fourth generation of them that hate me;
6 And shewing mercy unto thousands of them that love me, and keep my commandments.
Does not the bible tell of some people having evil spirits that caused sickness.
Luke 7:21 And in that same hour he cured many of their infirmities and plagues, and of evil spirits; and unto many that were blind he gave sight.
Luke 8:2
And certain women, which had been healed of evil spirits and infirmities, Mary called Magdalene, out of whom went seven devils,
Acts 19:12
So that from his body were brought unto the sick handkerchiefs or aprons, and the diseases departed from them, and the evil spirits went out of them.
I think this is very telling about not being able to cast out demons.
Acts 19:13 Then certain of the vagabond Jews, exorcists, took upon them to call over them which had evil spirits the name of the Lord Jesus, saying, We adjure you by Jesus whom Paul preacheth.
14 And there were seven sons of one Sceva, a Jew, and chief of the priests, which did so.
15 And the evil spirit answered and said, Jesus I know, and Paul I know; but who are ye?
16 And the man in whom the evil spirit was leaped on them, and overcame them, and prevailed against them, so that they fled out of that house naked and wounded.
17 And this was known to all the Jews and Greeks also dwelling at Ephesus; and fear fell on them all, and the name of the Lord Jesus was magnified.
18 And many that believed came, and confessed, and shewed their deeds.
19 Many of them also which used curious arts brought their books together, and burned them before all men: and they counted the price of them, and found it fifty thousand pieces of silver.
20 So mightily grew the word of God and prevailed.