1Cor.12 list nine (9) Gifts. #7 is Discerning of spirits (non-cap). One of the nine was chosen, because a book could be required for all combined.
This spirits is Not the Holy-Spirit. Many church-folk continue to make the same mistake that Eve, and Adam made in the Garden: if the spirit(s) doesn't startle, or invoke fear, the noun must be OK.
The LORD'S Command "do not touch the Tree of knowledge of Good, and evil" - we should all ask for, and be obedient to our Helper.
In Matt.17:21, Mark 9:29 it states "This kind." = More than one.
In Gen.3 - the Tree of Knowledge is in It's role as the Serpent: this word translated = Nachash, shinning one!×6
Lucifer was created by our LORD beautiful.
Mark 5:9 "legion" is an extreme demonic possession; which is one-eighty from smooth, deception of how most became victims. "This kind" via fasting/prayer, and assistance from the Holy Spirit, we can discern the spirit. Also if it is demonic, mental, or a person's "works of the flesh (Gal.5:19-21)" is being manifested.
Lucifer - is Antichrist: so are It's follower angels, and It's Children (Cain/Kenites,Tares, Naughty-figs, this Generation), and chronic/habitual - sinners. The Apostle John states "with an unction from the Holy-Spirit, you know."
"Only an Antichrist(spirit) will deny that Jesus is the Christ."