What is the difference, if any, between "unclean spirits" (Mark 5:2) and "demons" (Matthew 8:16)?

Question

I realized today that when I used the word "demons" I was thinking of evil spirits of all varieties, including human beings that work evil from the spirit world. But others believe the word "demon" only applies to non-human spirits such as fallen angels or perhaps evil deities of some sort.

In the Bible, Jesus refers to the "unclean spirit" [or spirits] that possessed the man who called himself Legion because he was tormented by so many evil spirits.

[Jesus said] “Come out of the man, you unclean spirit!” And Jesus asked him, “What is your name?” He replied, “My name is Legion; for we are many.” And he begged him eagerly not to send them out of the country. Now a great herd of swine was feeding there on the hillside; and they [the spirits] begged him, “Send us to the swine, let us enter them.” So he gave them leave. And the unclean spirits came out, and entered the swine; and the herd, numbering about two thousand, rushed down the steep bank into the sea, and were drowned in the sea. (Mark 5:9-13)

But elsewhere, Jesus heals various people possessed by "demons."

That evening they brought to him many who were possessed with demons; and he cast out the spirits with a word, and healed all who were sick.

I'd like to know more about how various Christian denominations and other religions view those who do evil in the spiritual realm, and especially: do they believe that the term "demons" includes evil spirits who were once human beings.

Rather than personal opinions, a good answer will provide the perspectives of several religions or experts. I would not rule out spiritualists in the latter category.

Answer 1

People are only ever human beings, never a different being even after death. A dog does not become a cat after death nor a pig become anything else after death.

We know this from Revelation 20 that the dead are to be born again by the breath of God to the form they had before but with immortality or to judgement and destruction by fire.

People are deceived by evil beings into believing that after death they may transform into another kind of creature. This transformation concept comes from Eastern mysticism.

We also know that Satan can change his appearance to look like an angel of light so it stands to reason that his minions may do the same and appear in the form of dead people to deceive the uneducated. These are demons/evil/unclean spirits spleen of in scripture.