How does one differentiate earth-realm demons/unclean spirits/devils and eternal-realm spirits (fallen angels)?
Succinctly put, one can tell an earth-bound unclean spirit from an eternal-realm spirit by the phenomenon of physical possession. Only earth-bound, unclean spirits will possess (i.e. inhabit) a human. Eternal-realm angels–even holy ones–may spiritually oppress (1 Sam 16:23, 19:9), adversely influence (1 Kings 22:22; 2 Chron. 18:21) or even kill (Exodus 12:23; 2 Kings 19:35; Is. 37:36) a human or group of humans, but earth-bound unclean spirits do the “infantry work” of possession.
To expound, I add:
Firstly, there are (fallen) angels, which–upon following Lucifer–left their first estate of holiness. But, their “estate” was not merely their essence of holiness. Some of these fallen angels later left the spiritual dimension (their “positions of authority as principalities and proper dwelling as eternal-realm spirits” —Jude 6) in order to cohabitate with human women in the physical dimension (apparently) irreversibly (Genesis 6). The essences of these physically-manifested fallen angels were chained in Tartarus after being eradicated physically by the flood, according to Jude. But, they (including Apollyon, a.k.a. Apollo, the Greek god; Shiva, the Hindu god of destruction [Prov. 15:11], Abaddon) will be let out again in varied form at the timing represented by Revelation 9:1-3, 11. [I believe this is beginning at CERN as we speak. see also this video].
Also, many reputable scholars gather from Genesis 6, Numbers 13, Raphaim (cited 33 times in Old Testament), etc. that these irreversible, physically-manifested fallen angels produced Nephilim—human/fallen angel hybrids. When these hybrid beings are physically killed, they are called demons/unclean spirits/devils… also referred to by some theologians as disembodied (evil) spirits. Their immaterial essences are relegated (limited) to dwell within our world, no access to 3rd Heaven. They wander to and fro, desiring a body—even an animal body—to inhabit (Matthew 12:43-45; Luke 11:24-25). And their prince (at least in the region of ancient Israel / Canaan) is called Baal-zebub, a pagan Philistine god. These earth-relegated, unclean spirits are those over which Christ gave his disciples direct authority… to cast them out of other humans, and some don’t come out except by our first praying and fasting (Matthew 10:1, 17:21; Mark 9:29; Luke 9:1). He gave us authority over no other spirits.
But, there are a remnant of fallen angels who did not incarnate, and these (along with Lucifer/Satan) ‘enjoy’ (Or, did enjoy, before the Advent of the Messiah) a primarily eternal-realm (3rd Heaven) existence, alongside the ability to manifest in and out of the physical (earth) realm. 1 Peter 5:8, Job 1:7 and 2:2 indicate Satan can go throughout the earth and to God’s throne (3rd Heaven). Revelation furthermore states that Satan himself will be kicked out of the heavenly realm one day, banished to earthly realm existence (Revelation 12:7-17).
Why will an eternal-realm, fallen angel not inhabit a human? An eternal-realm fallen angel has more important things to do, such as — Eph. 6: principalities (nation-wide rule), powers (region-wide rule), rulers of darkness (district or local area rule), spiritual wickedness in the ‘high places’ (specific locations* connected with evil worship due to performance of rituals, due to the location’s physics being conducive to inter-dimensional (spiritual, eternal realm) activity, or due to the fact there are places that are high like mountains or astrologically corresponding locations on earth, like ley lines).
A Christian has no authority whatsoever to speak against (audibly or inaudibly), rebuke or command angels, fallen or holy. Yes, it is true that we have been seated with Jesus Christ in the heavenly (eternal) realm, far above all principalities, powers, etc (Eph. 2:6). However, we Christians will not rule over angels (fallen or holy) until the time of Christ’s Millennial Kingdom (2 Tim. 2:12; 1 Cor. 6:2-3). Just as Christ’s kingdom is “already but not yet,” even so our ability to “command and rule” angels—much less Lucifer himself—is “already but not yet.” Now is the time we suffer with Christ in order to rule later. Just as he was subjected at the cross and arose victorious, we must be subjected now… and only when we are raised shall we be able to obtain the fullness of that for which we now have only a downpayment (Eph. 1:14; 2 Cor. 1:22; Romans 8:15-39). As the Romans 8 passage indicates, we are now subjected along with the rest of Creation until Christ’s enemies are made his footstool; until that time, our Lord Jesus waits at the Father’s right hand, and we wait here below (Luke 20:43, Ps. 110:1; Acts 2:35; Heb. 1:13; 10:13). Though Christ triumphed over Satan and all evil spirits at the Cross (Col. 2:14-16), there is a time coming soon when Satan will be put under our feet, even as he will be put under Christ’s feet (Romans 16:20).
What can believers do when eternal-realm angels oppress? We can “Submit yourself, then, to God. Resist the devil and he will flee from you” (James 4:7). Resist in James is the same word as “withstand” in Ephesians 6… a defensive term, not an offensive term. Christians are never to provoke or rebuke or ‘attack’ or command Satan or any eternal-realm fallen angel (Jude 1:9). Our appeals must be to God through Jesus, His Son. We must keep the Law of Orders. Submitting to God, Yielding to the Spirit and using the Armor of God as a defense is the only way to win at this present time. If we do these things, the enemy cannot get us off our course—God’s Life and grace. By rebellion, we fell; by submission we rise.
* Ancient Egyptian Manuscript Found That Describes “Multiple Gates” And Powerful Immortal Guardians At “Elaborate Architectonic Structure” SkywatchTV News, Sept. 22, 2015; Key word search: “High Places” (Blue Letter Bible)
A Summary on Demons vs. Fallen Angels by Mike Heiser | Naked Bible
Newsweek Special Edition: Life Beyond Earth—The Mission to Find the Answer, reviewed by Open Minds, October 19, 2017