Luke tells us that, after Jesus left Nazareth, he came to Capernaum and taught there on the Sabbaths (Luke 4:31), and on one Sabbath he was challenged by a man having an unclean spirit (Luke 4:33-34). Whatever one may think of unclean spirits, e.g. demons, spirits of the wicked dead or a fractured human spirit that is bent on destroying self or others, the New Testament reveals that Jesus and his disciples had authority over them. It makes no difference what they really are; the point in the New Testament is their power over men is undone by the authority of Jesus.
The fact is that the people of Capernaum were astonished with Jesus’ teaching, seeing that his word had great authority (Luke 4:32). He did not speak as did the scribes (rabbis) of his day. That is, the rabbis quoted one another and thereby established their authority. We do the same today. When we write a book about a particular subject, we often quote this or that author who wrote concerning the same subject matter, and we do this in an effort to show we are not alone in thinking as we do. Thereby we acquire some evidence of authority. The rabbis are seen doing this throughout the Talmud.
While Jesus taught in the synagogue at Capernaum on a certain Sabbath, he had to deal face to face with a man who had an unclean spirit. Jesus simply commanded the man to be silent and exorcised the demon that controlled his behavior (Luke 4:33-37). There was no stage acting; no performance to grade. He simply and forthrightly exorcised the demon. As a result the people wondered what kind of new doctrine this could be (Mark 1:27), because this was not how the ancient scribes handled similar problems. For example, in extra-biblical literature we find that a demon could be driven away with smoke made from the heart and liver of a fish:
Tobit 6:6-7 WEBA The young man said to the angel, “Brother Azarias, of what use is the heart, the liver, and the gall of the fish?” (7) He said to him, “About the heart and the liver: If a demon or an evil spirit troubles anyone, we must burn those and make smoke of them before the man or the woman, and the affliction will flee.
Another ancient demonstration of Jewish exorcism comes to us from Josephus:
I have seen a certain man of my own country, whose name was Eleazar, releasing people that were demoniacal in the presence of Vespasian, and his sons, and his captains, and the whole multitude of his soldiers. The manner of the cure was this: He put a ring that had a root of one of those sorts mentioned by Solomon to the nostrils of the demoniac, after which he drew out the demon through his nostrils; and when the man fell down immediately, he abjured him to return into him no more, making still mention of Solomon, and reciting the incantations which he composed. And when Eleazar would persuade and demonstrate to the spectators that he had such a power, he set a little way off a cup or basin full of water, and commanded the demon, as he went out of the man, to overturn it, and thereby to let the spectators know that he had left the man; and when this was done the skill and wisdom of Solomon were shown very manifestly. [Antiquities of the Jews 8.2.5]
All Jesus had was the word of God. This is all anyone is equipped with. Some may object and claim they have science, and through science man is able to cure himself of many ailments. While this is true, one must consider from where such knowledge (science) arose, and how did it come to be so lawful (predictable)? While many today presume a universe made up of matter that obeys predictable laws actually came out of chaos, no one is able to show that order by any means arises from chaos. It is merely presumed so. So, does order exist by the word of God (Genesis 1:3), or does it arise from chaos as modern science presumes?
Whatever men say about such things, all Jesus had was the word of God, the same word that created the universe and all that lay therein. With that authority alone, Jesus cast out the demon in Capernaum. While the scribes of his day placed their trust in the signs (viz. the smoke from the liver and the heart of a burning fish, or reciting incantations by some authority after which a cup of water was overturned by the fleeing demon), Jesus simply commanded the demoniac to be silent and the uncleanness to come out of him, and it was so (Luke 4:35).
 The traditional view is they are evil angels.
 Josephus refers to demons as spirits of the wicked (dead) in Wars of the Jews 7.6.3. When referring to a specific root he says: “it quickly drives away those called demons, which are no other than the spirits of the wicked, that enter into men that are alive…”