The Spirit clearly says that in later times some will abandon the faith and follow deceiving spirits and things taught by demons.
Things taught by demons.
Chew on that for a minu-et. Demons.
Keep in mind that we have a world full of people running all over the place saying "demons" don't exist. They are a figment of our imagination. But many of those same people will go around and proclaim the gospel of truth, of that "Jesus," as the one in which they believe, out one side of their mouth; and call the Savior a liar out the other.
Matthew 12 A House Divided
…27"If I by Beelzebul cast out demons, by whom do your sons cast them out? For this reason they will be your judges. 28"But if I cast out demons by the Spirit of God, then the kingdom of God has come upon you. 29"Or how can anyone enter the strong man's house and carry off his property, unless he first binds the strong man? And then he will plunder his house.
If the Lord Himself, and His followers and apostles spoke of demons, it nearly borders on the edge of blasphemy to say they don't exist ... because He said they do; and anything otherwise said calls the Lord Himself a liar. There's not much of a purpose in debating it. It is written in black & white and printed on paper, words of "Jesus" in red, believe it or not. The print is there in hard copy whether you choose to believe it or not. God isn't going anywhere because people choose not to believe in Him. [FYI: God/Allah/YHWH/Jehovah was never a "her."]
That said, what is it that Timothy meant when he said "The Spirit clearly says that in later times some will abandon the faith and follow deceiving spirits and things taught by demons.
What do "demons" teach? And how come there is no mention of a "demon" in the Old Testament, but only of "evil spirits," i.e., 1 Samuel 16, 18 and 19.
demon (n.) 
c.1200, from Latin daemon "spirit," from Greek daimon "deity, divine power; lesser god; guiding spirit, tutelary deity" (sometimes including souls of the dead); "one's genius, lot, or fortune;" from PIE *dai-mon- "divider, provider" (of fortunes or destinies), from root *da- "to divide" (see tide (n.)).
Used (with daimonion) in Christian Greek translations and Vulgate for "god of the heathen" and "unclean spirit." Jewish authors earlier had employed the Greek word in this sense, using it to render shedim "lords, idols" in the Septuagint, and Matt. viii:31 has daimones, translated as deofol in Old English, feend or deuil in Middle English. Another Old English word for this was hellcniht, literally "hell-knight."
The original mythological sense is sometimes written daemon for purposes of distinction. The Demon of Socrates was a daimonion, a "divine principle or inward oracle." His accusers, and later the Church Fathers, however, represented this otherwise. The Demon Star (1895) is Algol.
What do "demons" teach? And how come there is no mention of a "demon" in the Old Testament, but only of "evil spirits," i.e., 1 Samuel 16, 18 and 19. Given the Latin and Greek translations, and the interchangeable use with "spirit," "deity," "divine power," "lesser go," "tutelary deity," "unclean spirit," et al; what do these "spirits" and "tutelary deities" TEACH that Timothy seems to have a problem with when he says they will "abandon," and turn to "deceiving spirits"?
Luke 11:14 "Jesus" was driving out a "demon" that was mute. Demons apparently cause people not to be able to SPEAK. [Matthew 9:25]
John 10:21 Can a demon open the eyes of the blind? The people were observing the Lord causing blind people to see --and they didn't need healthcare insurance when He did it-- and they accused Him of being a demon, saying He was working 'magic' of some kind. He said to them, "all sins can be forgiven except the sin of blasphemy." In other words, calling GOOD works the works of evil is blasphemous to God. Two-fold application here: Demons BLIND people, and they had apparently been observed doing magic by those who accused the Savior.
Luke 4:35 Jesus was talking to a demon when He told it to shut up and come out of the man who was spewing out obscenities. The Lord recognized them instantly and told them to, in our own language, STFU and get the hell on gone...and they had no choice BUT to go. Are they entities that can hear and speak themselves? Do they curse and use foul language and whine and squeal every time they see the spirit of God in their presence? Apparently so.
Mark 7:26 A woman came to the Savior to beg him to drive the demon from her daughter. What was the daughter doing that was characteristic of the nature of the devil, or of demons? All it says is that her daughter had an "unclean spirit," doesn't really give us the full nature of what made the mother think her daughter had a demon; but apparently the Savior wasn't arguing that fact with her -- He told her "it isn't right to throw the Children's bread to the dogs." If you read the Bible at all, in any version, you know what she answered and the outcome of that story. But what we can glean from this is that demons can be driven out--as also happened in Luke 4:35.
What else?
Matthew 17:18 Apparently demons can be rebuked. Scolded.
Acts 19:15 Apparently, demons know people --and recognize their enemies, those whom they should fear and those whom they have no respect for whatsoever.
To make a very long story short, demons cause people to be physically and spiritually incapacitated and not able to fully function in their capabilities and capacities in which God created them. They have personalities, and they are able to hear and speak themselves.
The End.
So what do "demons" teach in this day and age?
We often speak of and hear of "the anti-Christ," and some have even been led astray by false teachings that there is "the" anti-Christ, and have even confused that spirit of the anti-Christ with some beast called "666." It is an error, once again.
Anti-Christ is anything and everything that is AGAINST Christ. There is no "the" anti-Christ, there is the SPIRIT OF the anti-Christ, according to what is written therein. [1 John 2:18,4:2-3 and 2:22]. That "spirit" is anything and everything that opposes Christ and does not confess that He is, in fact, the Son of God.
The "beast," and the "mark of the beast," as written in the Bible, has nothing to do with anti-Christ. To be sure, the beast of Revelation is anti-Christ, but so are YOU if you are against Christ and do not confess Him as Lord & Savior.
In a nutshell, anything and everything that is taught by those who do not teach righteousness, goodness, peace, comfort, love, ministry, and the Love of the Lord & Savior Himself is "taught by demons" and is a deceiving LYING spirit.
If the teaching BLINDS and does not reveal the truth, it is taught by a demon and a deceiving spirit.
It the teaching DEAFENS the ears of those who do not want to HEAR THE TRUTH, it is taught by a demon and a deceiving spirit.
If it cripples and weakens people and causes them to speak against the Lord and curse God, it is the teaching of a demon and a lying deceiving spirit.
If it does evil and proclaims itself to be good, even states that it is a "Christian," in the midst of that evil-doing and evil-mongering, it is the teaching of a demon, and a deceiving spirit.
So, considering that we now consider what is good to be evil and what is evil to be good, who is to say what is good and what is evil, or of a deceptive lying spirit?
Understand one thing ABOVE ALL ELSE ... GOD is NOT the Author of Confusion. [1 Corinthians 14:33]
YOU know in your own heart what is good and right and just. No one should have to tell you. It is given to you from birth, regardless of what the world may teach you later. If you did not know evil when you came into this world, you should not be listening to it whine and rail and carry on up a 'scandal' now. End Game.
James 3 / True Wisdom from Above
…14But if you have bitter jealousy and selfish ambition in your heart, do not be arrogant and so lie against the truth. 15This wisdom is not that which comes down from above, but is earthly, natural, demonic. 16For where jealousy and selfish ambition exist, there is disorder and every evil thing.…
I Corinthians 1:18-31