The Bible says the following about the power of demons.
Demons are able to enter and control both humans and animals.
For He had said to him, “Come out of the man, you unclean spirit!” Now there on the hillside a great herd of swine was feeding; So He gave them permission. And the unclean spirits came out and entered the swine; and the herd, numbering about two thousand, rushed down the steep bank into the sea, and were drowned in the sea (Mark 5:8,11,13).
These demons entered pigs after previously inhabiting humans.
More Than One
In some instances, more than one demon entered into the same victim. As with the case of the Gadarene demoniacs, there were a number of demons inhabiting those men.
We also have the testimony about Mary Magdalene.
Mary, called Magdalene, from whom seven demons had gone out (Luke 8:2).
Demons are able to inflict Mental Disease
Demons have the ability to inflict mental disease. One of the Gadarene demoniacs was described as follows.
for he had often been restrained with shackles and chains, but the chains he tore apart, and the shackles he broke in pieces; and no one had the strength to subdue him. Night and day among the tombs and on the mountains he was always howling and bruising himself with stones (Mark 5:4,5).
Demons are able to inflict Physical Infirmities
They are capable of imposing physical disease. We find demons inflicting the following illnesses upon people.
Demons have the ability to keep someone from using their powers of speech.
And when the demon had been cast out, the one who had been mute spoke; and the crowds were amazed and said, “Never has anything like this been seen in Israel” (Matthew 9:33).
According to Jesus, a woman had a physical deformity that was caused by a spirit.
He then emphasized this woman was suffering because of Satan.
And just then there appeared a woman with a spirit that had crippled her for eighteen years. She was bent over and was quite unable to stand up straight (Luke 13:11).
Blindness can also be a side affect of demonic activity.
Then they brought to Him a demoniac who was blind and mute; and He cured him, so that the one who had been mute could speak and see (Matthew 12:22).
Demons can affect both the physical and the spiritual nature of a person at the same time.
There are many possible causes for physical disease. Most times it is as simple as being exposed to some virus or bacteria, or the simple breaking down of the human body.
Judgment For Sin
Sometimes sickness is a judgment of God for sin.
For this reason many of you are weak and ill, and some have died (1 Corinthians 11:30).
Ultimately, we do not know why someone is sick. Therefore we should not assume that it is the judgment of God, or the activity of demons.
Demons can cause Erratic Behavior
We find the influence of demons causing people to behave strangely.
and said, “Lord, have mercy on my son, for he is an epileptic and he suffers terribly; he often falls into the fire and often into the water. And I brought him to Your disciples, but they could not cure him.” Jesus answered, “You faithless and perverse generation, how much longer must I be with you? How much longer must I put up with you? Bring him here to Me.” And Jesus rebuked the demon, and it came out of him, and the boy was cured instantly (Matthew 17:15-18).
Demons can harm unbelievers
Scripture gives us an example of unbelievers being harmed by demons.
Then some itinerant Jewish exorcists tried to use the name of the Lord Jesus over those who had evil spirits, saying, “I adjure you by the Jesus whom Paul proclaims.” Seven sons of a Jewish high priest named Sceva were doing this. But the evil spirit said to them in reply, “Jesus I know, and Paul I know; but who are you?” Then the man with the evil spirit leaped on them, mastered them all, and so overpowered them that they fled out of the house naked and wounded (Acts 19:13-16).
Demons can deceive Nations
Their work is not confined to individuals, but can include entire nations.
These are demonic spirits, performing signs, who go abroad to the kings of the whole world, to assemble them for battle on the great day of God the Almighty (Revelation 16:14).
Demons are free to roam on earth
Freedom on Earth Contrary to popular opinion, the Bible reveals that Satan and some of his demons were not confined to hell after their initial expulsion from heaven. Indeed, the Old Testament shows that, before the first coming of Yeshua the Messiah, Satan and the demons continued to have access to the very throne of God.
Now there was a day when the sons of God came to present themselves before the Lord, and Satan also came among them. Job 1:6
Then he showed me Joshua the high priest standing before the angel of the Lord, and Satan standing at his right hand to accuse him. Zechariah 3:1
Demons are not omnipresent nor do they have unlimited power. Their freedom is restricted, as is that of Satan, by their own nature and the power of God (Job 1:12; 2:6-7).
Demons are not all-knowing. Demons are fallen angels and angels don’t know everything (1 Peter 1:11-12).
There is no indication in the Bible that demons (or Satan for that matter) can read our thoughts. They are pretty good at predicting human behavior, however, because they are experts on human nature.
If some demons are obviously free to propagate evil in the world, there are Scriptures which indicate that a portion of the evil angels are currently restrained in a type of spiritual prison.
And the angels who did not keep their own position, but left their proper dwelling, he has kept in eternal chains in deepest darkness for the judgment of the great Day. Jude 6
For if God did not spare the angels who sinned, but cast them down to hell and delivered them into chains of darkness, to be reserved for judgment; II Peter 2.4 . . .
… he went and preached to the spirits in prison, 20 who formerly were disobedient, when once the Divine long suffering waited in the days of Noah, while the ark was being prepared, in which a few, that is, eight souls, were saved through water. I Peter 3:19
Clearly, the Bible teaches that there are at least two different groups of fallen angels; those who can move around freely and those who are currently locked up in “chains of darkness.”