What is the view of reformed churches on the subject of ghosts? Do they exist or do they not? If they exist, what are they: demons or spirits of dead people or both?
One of my “go to” sources for Reformed Protestant theology is Charles Haddon Spurgeon (1834-1892) who preached at the Metropolitan Tabernacle, a large Independent Reformed Baptist church in London. https://en.wikipedia.org/wiki/Metropolitan_Tabernacle This link provides access to 3,653 of his sermons: https://www.spurgeongems.org/spurgeon-sermons/
I searched on key words such as “ghosts”, “dead spirits” and “demons” but all I could find was a reference to Deuteronomy 18:11 which says that anyone who consults with the dead is detestable to the Lord. From there I searched on the Witch of Endor because that is what King Saul did – he consulted with a medium to bring up the spirit of the dead prophet Samuel. This cost Saul his life “because he was unfaithful to the LORD; he did not keep the word of the LORD and even consulted a medium for guidance” (1 Samuel 28:1-25; 1 Chronicles 10:13-14). Another Bible reference about demons and false spirits warns against people who “will abandon the faith and follow deceiving spirits and things taught by demons” (1 Timothy 4:1).
Here are a few partial quotes from sermons that mentioned Saul consulting the Witch of Endor in an attempt to seek guidance from the dead prophet Samuel:
Charles Spurgeon Sermon 457 - Religion – Reality (page 1 paragraph 2): Regarding ceremonial religion: You might as well go to the Witch of Endor for divine grace as to a priest; and if you rely upon words, the “Abracadabra” of a magician will as certainly raise you to heaven, or rather sink you to hell, as the performances of the best ordained minister under heaven! ...All ceremonial religion, no matter how sincere, if it consists in relying upon forms and observances, is a vain thing. https://www.spurgeongems.org/sermon/chs457.pdf
Charles Spurgeon Sermon 439 – The Danger of Doubting (page 3, paragraph 2): Saul was seeking a miserable witch of Endor, to raise Samuel from the dead... When the Witch of Endor brought up the spirit of Samuel, he said to Saul “Why do you consult me, now that the Lord has turned away from you and become your enemy?” (1 Samuel 28:16) https://www.spurgeongems.org/sermon/chs439.pdf
Charles Spurgeon Sermon 2179 – God Fighting Sin (page 3, paragraph 1): Regarding preachers who leave God: If God will not help him he is in the same plight as Saul, the son of Kish. He will try music first, and if that does not render him aid, he will go to the witch of Endor, now called “modern theology,” and ask assistance there! God have mercy upon us, if we ever do that! https://www.spurgeongems.org/sermon/chs2179.pdf
I concede that these quotes do not answer your question, so I looked elsewhere and have put together a brief overview of Protestant thought based on what the Bible says:
The Bible makes it abundantly clear that there are spirit beings, both good and evil. But the Bible negates the idea that the spirits of deceased human beings can remain on earth and “haunt” the living. According to 2 Corinthians 11:14-15, demons masquerade as “angels of light” and as “servants of righteousness”: “And no wonder, for even Satan disguises himself as an angel of light. So it is no surprise if his servants, also, disguise themselves as servants of righteousness. Their end will correspond to their deeds.” Appearing as a “ghost” and impersonating a deceased human being definitely seem to be within the power and abilities that demons possess. Whether it is called a ghost, a ghoul, or a poltergeist, if there is genuine evil spiritual activity occurring, it is the work of demons. What about instances in which “ghosts” act in “positive” ways? What about psychics who claim to summon the deceased and gain true and useful information from them? Again, it is crucial to remember that the goal of demons is to deceive. If the result is that people trust in a psychic instead of God, a demon will be more than willing to reveal true information. Even good and true information, if from a source with evil motives, can be used to mislead, corrupt, and destroy. https://www.gotquestions.org/ghosts-hauntings.html
Some people hold the idea that unclean spirits (Greek ‘akathartos’) or demons are deceased humans who may or may not have been evil while alive. However, we know the unclean spirits mentioned in the Bible are not referring to the dead, for several reasons. One, humans are never called “spirits” when the word spirit is used as a stand-alone term, without a possessive. In Scripture, men are said to have a spirit/soul (saying “his spirit” in Proverbs 25:28 and 1 Corinthians 5:5), but men are not called “spirits.” Another reason is that, once a person dies, he immediately goes either to eternal life with the Lord or to eternal darkness in hell (Hebrews 9:27; 2 Corinthians 5:6–8; Matthew 25:46). Human spirits, therefore, do not and cannot wander on earth in their spirit bodies. Any unclean spirit that wanders around, taking up residence in places or people or interacting with people in any way, is a fallen angel—a demon (Matthew 12:44). All unclean spirits mentioned in Scripture are demons, and all demons are definitely unclean, unholy, impure, evil spirits doomed to an eternity in hell (Matthew 25:41). https://www.gotquestions.org/unclean-spirits.html
Looking into the history of belief in ghosts I found some interesting information on how ghosts are viewed by Catholics, Protestants, Hindus, Buddhists and Muslins. It’s quite revealing:
In Medieval Catholic Europe, ghosts were assumed to be the tormented souls of people suffering for their sins in purgatory. But during the Protestant Reformation, since most Protestants believed that souls went immediately to heaven or hell, paranormal activity was thought to be the work of angels, demons or other decidedly nonhuman supernatural beings. For Hindus, ghosts are the souls of individuals who suffered a violent death or of people who were not accorded the appropriate and required death rituals. Buddhist ghosts are reincarnated individuals who may be sorting out bad karma. Muslims don’t believe that dead people can return as ghosts, so if a Muslim thinks he’s encountered a ghost, it’s thought to be the work of Jinn – beings that contain a mix of spiritual and physical properties, whose intentions can be malevolent or benevolent depending upon the situation. There are several other religions, such as Jehovah’s Witnesses, that also believe ghostly apparitions are demons in disguise rather than the souls of deceased people. It appears people across eras, religions and cultures have always been curious about a spiritual world that exists behind the curtain of death. https://theconversation.com/how-the-god-you-worship-influences-the-ghosts-you-see-84163
As a Swiss Reformed Protestant theologian, Ludwig Lavater rejected the idea of Purgatory (an intermediary place where souls of the newly dead that are destined for heaven go, until they are pure enough to ascend) as an outdated Catholic concept. This greatly complicated the idea of ‘ghosts’, often thought to be visitations by human souls that were not at rest, such as those who died unbaptized or in tragic or violent circumstances. Without Purgatory, ghosts could only be visitations from Heaven or Hell. Lavater felt they were more likely to have come from Hell, and this meant that many ghosts were demonic and their requests dangerous: they could be trying to lure humans into damnation, for example by persuading them to commit murder or suicide The Protestant rejection of Purgatory also made the subject of ghosts and their nature controversial and political as it became part of sectarian religious debate. https://www.bl.uk/collection-items/of-ghosts-and-spirits-walking-by-night-by-ludwig-lavater-1572
Finally, this Wikipedia article looks into biblical references to mediums, summoning the dead and ghosts: https://en.wikipedia.org/wiki/Ghost#Judaism_and_Christianity
The difficulty in answering this question is in trying to find anything published by Reformed Protestants on the subject of "ghosts". They have plenty to say about the Holy Ghost or Holy Spirit, but any other sort of "spirit" or "ghost" does not come from God.