DEFINITION OF A DEMON.

A Demon is an wicked spirit, entity or creature, sometimes called "fallen angel," having superhuman powers. Demons as such were not created by GOD. The first to make himself one of them was Satan, who became the ruler of other angelic sons of GOD who also made themselves demons.
In classical Greek, demons were considered as intermediary spiritual beings which moved up and down or be either good of bad.
Demons feature prominently as adversaries in the Ministry of JESUS, and as agents of Satan they must be overcome if JESUS's Ministry is to be successful. JESUS himself speaks of having come to plunder Satan's goods. Luke 11:21-22. Expulsion of demons from people is one means by which JESUS does so in Matthew 12: 27-28
Matthew11 says: When JESUS had finished instructing His 12 disciples, HE went on from there to Teach and Preach in their cities. Now when John (the Baptist) heard in prison about the Deeds of The CHRIST, he sent word by his disciples and said to HIM, "ARE YOU HE WHO IS TO COME, or shall we look for another?" And JESUS answered them, "Go and tell John what you hear and see: the blind receive their sight and the lame walk, lepers are cleansed and the deaf hear, and the dead are raised up, and the poor have good news preached to them. And blessed is he who takes no offense at ME." As they went away, JESUS began to speak to the crowds concerning John: "What did you go out into the Wilderness to behold? A reed shaken by the Wind? Why then did you go out? To see a man clothed in soft raiment? Behold, those who wear soft raiment are in king's houses. Why then did you go out? To see a prophet? Yes, I tell you, and more than a prophet. This is he of whom it is written, 'Behold, I send My Messenger before your face, who shall prepare the Way before YOU.'" 11:1-10
Truly, I say to you, among those 'born of women' there has risen no one greater that John the Baptist; yet he who is least in the kingdom of heaven is greater than he.  11: 11-12
From the Days of John the Baptist until now the Kingdom of Heaven has suffered Violence, and Men of Violence take it by Force. For all the Prophets and the Law prophesied until John; and if you are willing to accept it, he is Elijah who is to come. 11: 12-15
Matthew12 says: At that Time JESUS went through the Grain Fields on the Sabbath; his disciples were hungry, and they began to pluck heads of grain and to eat. But when the Pharisees saw it, they said to HIM, "Look, Your disciples are doing what is Not Lawful To Do on the Sabbath." HE said to them, "HAve you not read what David did, when he was hungry, and those who were with him: how he entered the HOUSE OF GOD and ate bread of the PRESENCE, which it was not lawful for him to eat nor for those who were with him, but only for the Priests? Or have you not read in the law how on the Sabbath the Priests in the Temple profane the Sabbath, and are guiltless? I tell you, SOMETHING GREATER than the Temple IS HERE. And if you had known WHAT THIS MEANS, 'I desire MERCY, and NOT SACRIFICE,' you would not have condemned the guiltless. For the SON of MAN IS LORD OF THE SABBATH. " 12: 1-8
And JESUS went on from there, and entered their synagogue. And behold, there was a man with a withered hand. And they asked HIM, "It is Lawful to Heal on the Sabbath?" so that they might accuse HIM. Jesus said to them, "What Man of You, if he has one sheep and it falls into a pit on the Sabbath, will not lay hold of it and lift it out? Of how much more value is a Man than a sheep! So It Is Lawful To Do Good on the Sabbath." Then JESUS said to the man, "Stretch out your hand." And the man stretched it out, and it was restored, whole like the other. But the Pharisees went out and took counsel AGAINST HIM, how to destroy HIM. Jesus aware of this, withdrew from there.  12: 9-15
In NOAH's Days these disobedient and wicked angels materialized by marrying the daughters of men and fathering a hybrid generation known as NEPHILIM, and then dematerialized when the Flood came. (Genesis 6). However, upon returning to the spirit realm they did not regain their lofty original position, for Jude 6 says: "The angels that did not keep their original position but forsook their own proper dwelling place HE has reserved with eternal bonds under dense Darkness for the Judgment of the Great Day." So it is in this condition of dense spiritual Darkness that they must now confine their operations.
Simeon Peter says in his 2nd Letter: To those who have obtained a FAITH OF EQUAL STANDING WITH OURS IN THE RIGHTEOUSNESS OF OUR GOD and SAVIOR JESUS CHRIST: May Grace and Peace be multiplied to you in THE KNOWLEDGE OF GOD and of JESUS our LORD.
His DIVINE POWER has granted to us all things that pertain to LIFE and GODLINESS, through the KNOWLEDGE OF HIM who called us to His Own Glory and Excellence, by which HE HAS GRANTED TO US HIS precious and very great PROMISES, that THROUGH THESE you may ESCAPE FROM the CORRUPTION that is IN THE WORLD because of passion, and BECOME PARTAKERS OF THE DIVINE NATURE. For this very reason make every effort to supplement your FAITH with VIRTUE, and VIRTUE with KNOWLEDGE, and KNOWLEDGE with SELF-CONTROL, and SELF-CONTROL with STEADFASTNESS, and STEADFASTNESS with GODLINESS, and GODLINESS with BROTHERLY AFFECTION, and BROTHERLY AFFECTION with LOVE. For if these things are yours and abound, they keep you from being ineffective or unfruitful in the KNOWLEDGE of our LORD JESUS CHRIST. For whoever LACKS these things IS BLIND and SHORTSIGHTED and has FORGOTTEN that HE WAS CLEANSED from his OLD SINS. Therefore, be more zealous to confirm your call and election, for IF YOU DO this YOU WILL NEVER FALL; so there will be richly provided for you an entrance into the eternal kingdom of our LORD and SAVIOR JESUS CHRIST.  2 Peter 1: 1-11
We did not follow cleverly devised myth when we made known to you THE POWER and COMING of OUR LORD JESUS CHRIST, but we were eyewitnesses of His Majesty. For when HE received HONOR and GLORY from GOD the FATHER and the VOICE was borne to HIM by the Majestic Glory, "This is My Beloved Son, with Whom I Am Well Pleased," we heard this Voice borne from Heaven, for we were with HIM on the Holy Mountain. And we have the Prophetic Word made more sure. You will do well to PAY ATTENTION TO THIS as to a lamp shining in a dark place, until the day dawns and the morning start rises in your hearts.  2 Peter 1: 16-19
First of all YOU MUST UNDERSTAND this, that no Prophecy of Scripture is a matter of one's own interpretation, because NO PROPHECY ever came by the IMPULSE OF MAN, but men moved by the HOLY SPIRIT spoke from GOD. 2 PETER 1: 20-21
But False Prophets also arose among the people, just as there will be False Teachers among you, who will secretly bring destructive heresies, even denying the Master Who Bought them, bringing upon themselves swift destruction. And many will follow their licentiousness, and because of them THE WAY OF TRUTH WILL BE REVILED. And in their greed they will exploit you with FALSE WORDS; FROM of OLD their CONDEMNATION HAS NOT BEEN IDLE, and their destruction has not been asleep. For If GOD did not spare the angels when they sinned, but cast them into HELL and committed them to pits of nether gloom to be kept until the Judgment; if HE did not spare the Ancient World, but preserved NOAH, a herald of RIGHTEOUSNESS, with 7 other persons, when HE brought a Flood upon the World of the Ungodly; if by turning the Cities of Sodom and Gomorrah to ashes HE condemned them to extinction and made them an example to those who were to be ungodly; and if HE rescued RIGHTEOUS LOT, greatly distressed by the licentiousness of the Wicked (for by what that righteous man saw and heard as he lived among them, he was vexed in his righteous soul day after day with their lawless deeds), then the LORD knows how to rescue the godly from trial, and to keep the unrighteous under punishment until the Day of Judgment, and especially those who indulge in the lust of defiling passion and despise authority. 2 Peter 2: 1-10
According to early Christians, demons oppress attacking people from without, or possess entering an individual's body and attacking it from within. They cause diseases and sickness of all kind, although not all sickness may be attributed to them. Mark 1:32; 2:10-12. Most of the time a demonic activity in a person's life would be obvious. Mark 5: 1-19; 9:14-29
The purpose of all such demonic activity is to turn people against GOD. Demon influence in human affairs is no less manifest today. Prophetic warning is given concerning accelerated demon activity on the earth. Revelation 1:1 says, "Down the great dragon was hurled, the original serpent, the one called Devil and Satan, who is misleading the entire inhabited earth; he was hurled down to the earth, and his angels (demons) were hurled down with him."  Revelation 12: 9, 12 says, "Woe for the earth and for the sea (Flesh and Mind), because the Devil has come down to you, having great anger, knowing he has a short period of Time." Christians must, therefore, put up a hard fight against these unseen wicked spirits.