There are many things that may be said concerning the Devil and his legions of fallen beings. But have no doubt that the Devil and his host are very real. They are conscious, intelligent, evil, corrupt, sinful, depraved entities who are allied with Satan. Besides the various Old Testament references to them and their master, Jesus spoke of them (Matt. 12:27-28) and even sent the seventy out to deal with some of them (Luke 10:17). The apostle Paul spoke of them (Acts 16:16-18; cf. 2 Cor. 2:11-14; 2 Tim. 2:26; 2 Thess. 2:9), as did James (Jas. 2:19), and Peter (1 Pet. 5:8) as well. So, a malicious demonic realm of darkness does exist.

However, it must be emphasized that the power of all fallen beings, including the Devil himself, is limited (cf. Psa. 76:10). God is sovereign over all things — including every fallen being. They cannot do anything they desire; rather they are under the ultimate sovereign control of God himself (cf. Job 1:8-12; 2:1-7). But as Scripture informs us, God is sovereign and can choose to use evil beings to carry out his eternal decrees, plans and purposes, demonstrating that he remains the supreme ruler over all the universe — including all fallen beings. And while Christians may certainly be tempted (1 Thess. 3:5; cf. Matt. 4:3; 1 Cor. 10:13; 2 Cor. 11:3) and otherwise be attacked by such entities (i.e., Job), ultimately, they have nothing to fear (cf. Luke 11:20; 22:23; Rom. 8:31-39). Why? Because Satan's entire kingdom was delivered a defeating blow at the cross (John 12:31; Col. 2:15; cf. Gen. 3:15). (Please see "How does temptation work?" below.)

While definitely not comprehensive, here are a few notes:

I believe the Bible teaches that there is a demonic realm with a structured hierarchy (cf. Eph. 6:12). Scripture doesn't directly inform us of how Satan and his host fell, but it is normally taught that when he fell from heaven (cf. Isa. 14:12-14; Ezek. 28:12-19), he took a host of other fallen beings with him (cf. Luke 10:18; Rev. 12:3-4). (Please see "Who is the Prince of Tyre" below.) Satan's fallen host are referred to by numerous names in the Bible; such as "unclean spirits" (cf. Matt. 12:43, Greek: akatharton pneuma), "evil spirits" (cf. Acts 19:13-16 — pneumata ta ponera), "deceitful spirits" (cf. 1 Tim. 4:1 — pneumasin planois), "lying spirits" (cf. 1 Kings 22:22-23 — LXX, pneuma pseudes), and "demons" (cf. Deut. 32:17; Psa. 106:37; Matt. 4:24; 1 Tim. 4:1 — daimonion). Some of these names may be interchangable, but these entities are eternally destined for hell (Matt. 25:41). (Please see "Wasn't hell made for the Devil and his angels and not real people?" below.)

In the Bible, it appears that various fallen beings focus, influence, and manifest their evils on different areas of life. Though all sickness is not linked to demons (cf. Matt. 4:24) some was; such as physical Illness (Matt. 9:33), deformity (Luke 13:11, 16), mental impairment (Mark 5:4-5), blindness (Matt. 12:22), epilepsy (Matt. 17:15-18), and even physical possessions (Matt. 12:22-29; Luke 8:2). It's also notable at least some fallen beings appear to be associated with astrology (cf. Isa. 47:12; cf. Acts 7:43), divination (Deut. 18:10; 1 Sam. 15:23; 2 Kings 17:17; Jer. 14:14; Acts 16:16), magic (cf. Gen. 41:8; Exod. 7:11, 22; 2 Tim. 3:8), necromancy (communication with the dead; Lev. 19:26, 31; Jer. 27:9-10), sorcery (Exod. 22:18), and witchcraft (Gal. 5:19-21). They spread false doctrine (1 Tim. 4:1-3), false religions, culture, and even influence politics (Rev. 16:14).

Beginning in Genesis 3, fallen beings appear to begin their destructive trek throughout redemptive history. They were instrumental in original sin (Gen. 3) and the death of Christ (cf. Acts 2:23; 4:27-28). And they were rather shocked by his resurrection and what it meant for them (1 Cor. 2:8; Col. 2:15), so they spun some more lies (cf. Matt. 27:62-66; 28: 4, 11-15). They are also involved in end time events (cf. Rev. 16:13-16, et. al.). Some of their functions may differ, however, in all cases these emissaries of Satan are malevolent beings who hate God, all other beings (esp. Christians), and constantly attempt to subvert the kingdom of light (Acts 26:18; Col. 1:12-13; cf. John 10:10). (Please see "List of False Religions" below.)

God at times uses evil to accomplish his purposes (WCF 5.4) and even uses them to send judgment when he has decreed that it is righteous and just to do so (cf. Judg. 9:23; 1 Sam. 16:14-15; 1 Kings 22:20-23). (Please see "Does God Use Evil to Accomplish His Purposes?" and "Evil and God?" below.) But just because God is sovereign over evil and all evil beings, does not negate the Christian's responsibility to actively resist both (Eph. 6:13; 1 Pet. 5:8-9; Jas 4:7). (Please see "God's Sovereignty and Our Responsibility?" below.)

These fallen beings are opposed to all that is holy, righteous and good (cf. Rom. 7:12). They hate biblical love, grace, faith, and mercy — and their Author! Though at times they appear as angels of light (cf. 2 Cor. 11:14), they are ultimately seeking but to steal, kill, and destroy (cf. John 10:10). So, these fallen evil beings cannot be regarded with neutrality. The Christian is commanded to fight against all kinds of evil (cf. Eph. 6:13-14). He must take a stand! However, he may still rest in peace (cf. Psa. 3:5; 4:8; Prov. 3:24; Phil. 4:7) knowing that "if God is for us, who can be against us?" (Rom. 8:31-39).

Related Topics

How does temptation work?
Who is the Prince of Tyre?
Wasn't hell made for the Devil and his angels and not real people?
Does God Use Evil to Accomplish His Purposes?
Evil and God?
God's Sovereignty and Our Responsibility?
List of False Religions
Is there one God or many gods?