Many people believe in ghosts, but are they actually ghosts or demons?
Demons are spirit beings who fell from being holy to becoming corrupt. They decided to rebel with Lucifer, later Satan, and are considered fallen angels for that reason. They do the bidding of the Devil himself, likely following his agenda and his primary target are Christians and Christian leaders in the church. Since God knows ever star by a given name, even though there are untold billions and billions of them, it is very likely that God has a name every for angel too. He gave names to the Archangels, Michael and Gabriel, and He names Lucifer, whose name means “light bringer” or “morning star” (Psalm 147:4), now he is the Prince of Darkness. In a few cases, these demons actually gave their names when asked by Jesus because He had authority over them. They had no choice but to obey. In one case, “Jesus asked him, “What is your name?” He replied, “My name is Legion, for we are many” (Mark 5:9). Why Legion? Most of the Roman Legions had approximately 4,800 men, so this was not only one demon but thousands of them. That explains the possessed man’s supernatural strength. 
Saul Consults a Medium
Some claim communicating with the dead is permissible because King Saul consulted with Samuel. In the first place, do we really want to use Saul as an example? He had fallen away from God and was even breaking the commandment to consult with the dead, so what he was doing was not condoned by God at all. In fact, it was a grievous sin and God calls this idolatry (Deut 18:10-12; Isaiah 8:19), but even more so, Saul didn’t actually communicate with the Prophet Samuel because even the witch at Endor who was a medium was shocked that someone actually appeared and spoke (1 Sam 28)! The appearance may have looked like the Prophet Samuel, but that’s why they call them “familiar spirits,” since they can sometimes appear to be someone they’re familiar with. Whether this was Satan (who is a deceiver and liar) or a demon imitating Samuel or it actually was Samuel, the Bible does not say precisely. We do know this: the Bible tells us, “Don’t consult with the dead.” That’s an abomination to God.
One pastor visited a bed and breakfast; one that was supposedly haunted. The hosts and guests all accepted the fact that the ghost lived there and they weren’t really afraid of it, but when the pastor said, “The Bible says that there are familiar spirits in the world who people think are ghosts but they are actually demonic spirits who are familiar with the dead.” That’s why they call them “familiar spirits.” The host of the bed and breakfast didn’t like that explanation and stuck with the word ghost. It was a lot friendlier she thought. But these ghosts are anything but like Casper the Friendly Ghost! The Bible says that after death comes the judgment (Heb 9:27), and like Lazarus and the Rich Man taught us, that chasm is too far for anyone to come back from the dead…even to warn family about the realities of hell (Luke 16:27-28).
The Bible has a lot to say about familiar spirits, and in particular, those who try to communicate with the dead. They might actually hear from someone, but it’s not the “someone” who they think it is!! It is surely not the deceased but might actually be an evil spirit who is familiar with the dead person. In knowing the deceased person they seek to communicate, these demons can easily imitate the voice and even what they looked like perhaps, but that’s not Grandpa! The Word of God says “Do not turn to mediums or necromancers; do not seek them out, and so make yourselves unclean by them: I am the Lord your God” (Lev 19:31). People have a craving for things like trying to communicate with their lost loved ones, so they “Inquire of the mediums and the necromancers who chirp and mutter,” should not a people inquire of their God? Should they inquire of the dead on behalf of the living” (Isaiah 8:19)? The answer to that rhetorical question is obvious. No!
Fruits of the Flesh
Some of the fruits of the flesh or fruits of those who are not saved include “Idolatry, sorcery, enmity, strife, jealousy, fits of anger, rivalries, dissensions, divisions” (Gal 5:20), so God equates sorcery or divination as being equal to all these other detestable sins. I believe this is still in effect as much as it was in the ancient days for the people of God that “There shall not be found among you anyone who burns his son or his daughter as an offering, anyone who practices divination or tells fortunes or interprets omens, or a sorcerer” (Deut 18:10). They may not be putting their children in the fire anymore, but they’re playing with fire nonetheless.
Playing With Fire
Sceva, “a Jewish chief priest” (Acts 19:14) had seven sons “who went around driving out evil spirits” (Acts 19:13), but they were playing with fire, not realizing they were toying with supernatural beings who had supernatural powers. When they tried to cast out a demon in one man, they said, “In the name of the Jesus whom Paul preaches, I command you to come out,” but one day they messed with the wrong demon! The demon viciously turned on them and the demon-possessed man “jumped on them and overpowered them all. He gave them such a beating that they ran out of the house naked and bleeding” (verse 16). The seven sons of Sceva were no match for the demonic powers of hell under Satan’s reign. Their make-shift “exorcists” business probably ended that day.
Why were the seven sons no match for the demon? To begin with, they didn’t actually have the authority of Jesus to do that. Even more, they didn’t even have a personal relationship with Him, most likely. Jesus gave the 70 other disciples the same authority that He Himself had over demons. The seven sons never had that. Jesus would have had to give them the authority to cast out demons (Luke 10), or they’d never had any success at all. Trying anything for God apart from Christ comes to nothing. The 70 were able to cast out demons, but only because Jesus had given them that authority and the demons must have known that too. The demons told the sons of Sceva that they knew Paul and Jesus, but “Who in the world are you?!” The point being, the demons know those who belong to Jesus and those who don’t, and if you’re not saved, don’t even try to mess with seeking out ghosts or casting out demons. You have no idea what might happen. Unless the 70 disciples Jesus sent out had been given His authority, they would have been received just as badly as Sceva’s sons were. 
God is serious about trying to communicate with the dead or using fortune tellers, reading tarot cards, or trying to communicate with ghosts. This area is off limits to Christians. The New Testament does not teach that Jesus’ disciples should seek out and try and cast out demons, but to share the Word of God about the Son of God, and then let the “strong man” take possession of Satan and cast Him out by the power of the Holy Spirit. The Spirit has authority over Satan and his demons, and when someone is born again, they know have God living in them! That means, there’s no room at the inn for Satan or his demons, so remember, God warns that “If a person turns to mediums and necromancers, whoring after them, I will set my face against that person and will cut him off from among his people” (Lev 20:6). This was such a serious crime that “A man or a woman who is a medium or a necromancer shall surely be put to death. They shall be stoned with stones; their blood shall be upon them” (Lev 20:27). If that was serious enough to warrant death, we don’t even need to go there.
Here is some related reading for you: What do Christians Believe About Ghosts? 
Resource – Scripture quotations are from The Holy Bible, English Standard Version® (ESV®), Crossway Bibles. (2007). ESV: Study Bible: English standard version. Wheaton, Ill: Crossway Bibles. Used by permission. All rights reserved.