The bible is very clear from start to finish that there are angelic beings, some of which have rebelled against God. In Job, Satan came to God, and spoke with Him, asking God to test Job, to put sickness on him, and destroy his family. In the New Testament Satan is spoken of as a real being. Jesus went around casting out demons, he even said when a devil leaves a person it does XZY. Revelation the last book of the bible speaks of Satan and his angels, and their ultimate doom in hell. Every where the bible says Satan is a real being.
Here are some examples, but there are 100s more showing Satan is considered in the bible as a real entity not an idea or thought:
THE DEVIL SPOKE TO GOD, CAME TO GOD, IS NOT AN IDEA
Job 1:6-9 Now there was a day when the sons of God came to present themselves before the LORD, and Satan came also among them. And the LORD said unto Satan, Whence comest thou? Then Satan answered the LORD, and said, From going to and fro in the earth, and from walking up and down in it. And the LORD said unto Satan, Hast thou considered my servant Job, that there is none like him in the earth, a perfect and an upright man, one that feareth God, and escheweth evil? Then Satan answered the LORD, and said, Doth Job fear God for nought?
JESUS SAID THE DEVIL AND HIS ANGELS WOULD BE SENT TO HELL
Mat 25:41 Then shall he say also unto them on the left hand, Depart from me, ye cursed, into everlasting fire, prepared for the devil and his angels:
DURING HEALING JESUS CAST OUT DEVILS
Mar 7:29 And he said unto her, For this saying go thy way; the devil is gone out of thy daughter.
Luk 4:35 And Jesus rebuked him, saying, Hold thy peace, and come out of him. And when the devil had thrown him in the midst, he came out of him, and hurt him not.
Luk 13:16 And ought not this woman, being a daughter of Abraham, whom Satan hath bound, lo, these eighteen years, be loosed from this bond on the sabbath day?
JESUS SPOKE OF THE DEVIL DECEIVING PEOPLE
Luk 8:12 Those by the way side are they that hear; then cometh the devil, and taketh away the word out of their hearts, lest they should believe and be saved.
JESUS WAS TEMPTED BY THE DEVIL AND TOLD HIM TO LEAVE
Mat 4:10 Then saith Jesus unto him, Get thee hence, Satan: for it is written, Thou shalt worship the Lord thy God, and him only shalt thou serve.
JESUS SAID THE DEVIL WISHED TO SIFT PETER
Luk 22:31 And the Lord said, Simon, Simon, behold, Satan hath desired to have you, that he may sift you as wheat:
THE APOSTLES REFERRED TO THE DEVIL
1Jn 3:8 He that committeth sin is of the devil; for the devil sinneth from the beginning. For this purpose the Son of God was manifested, that he might destroy the works of the devil.
AGAIN JOHN IN REVELATION MENTIONS THE DEVIL AS REAL
Rev 20:7 And when the thousand years are expired, Satan shall be loosed out of his prison,
Rev 12:9 And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him.