The Enemy
There exist three enemies of humankind: the world, the flesh, and the devil. One of the advantages of the believer in Jesus is that he is not subject to the authority of any of these. That isn't to say that believers are immune to the temptations thereof - even Jesus was tempted1. I'll briefly cover them in order.
The World
In "Christianeze", this is referring to any religions, belief systems, philosophies, and structures of society and government which are contrary to the work of God. Sometimes this even includes churches. Oppressive governments, pornography and the sex trade, hate groups, violent criminals, and the like are all clear examples of this usage of the word "world". But it also applies to more subtle things: books, music, movies, web sites, and social media (all, in themselves, neither good nor bad) when they are contrary to the work of God. Perhaps it is a TV show that casts slander on the character of God; or a book that misrepresents history to tell a lie about Jesus; or a movie that makes sin look appealing. All of these things are working to hide from people the goodness of God, the need for Jesus, and the consequences of selfishness. Obviously, not all TV shows, movies, music, or books are "bad". But remember that we are in a spiritual battle, and the enemy uses propaganda of all forms to promote a false narrative that directs people down the path to death, whereas the Spirit uses the truth to direct people down the path to life.
The Flesh
This is a reference to the inner person - the self-centered heart that desires to place oneself above others and even above God Himself. Although the believer has the Holy Spirit providing the power to overcome the flesh, we are all subject to its pull and temptation this side of heaven. Those who are not followers of Jesus do not have the Spirit living in them and are slaves to the flesh. Perhaps most people don't give in fully to the flesh, but at one point, everyone has given in to some part of it - if we hadn't we wouldn't be sinners in need of Jesus. Jesus is the only man who never gave in to any of the enemies of our souls.2 The World provides temptations that tantalize our flesh and, in turn, people living according to their flesh help to shape the world. It is only the influence of a loving God that keeps people from being worse than they are.
The Devil
The Devil is an actual being3 - more powerful than humans, but less powerful than God. He has to ask God for permission to do certain things.4 As a single being, the Devil cannot be in multiple places at the same time. In fact, I suspect he is mainly operating in the big power centers of world governments and business. The average person, like you and I, are simply not important enough to warrant any interest from him. However, he has his own set of followers - the fallen angels, also known as demons, or devils (small "d"). For the sake of clarity, I will henceforth use the term "Satan" to refer to the Devil and use "Devil" loosely as a reference to the forces of evil at the beck and call of Satan.
The Devil uses two basic strategies - whichever one works best for each individual. The first is to pretend he doesn't exist. Those who scoff at the idea of Satan have fallen prey to this first strategy. He is not merely a bad guy from fairy tales, medieval writing, or a symbolic representation of evil. The second strategy is to get people to obsess over him - either in fear or in titillation. If you see the Devil hiding under every rock, or spend inordinate amounts of time thinking about him, you have fallen prey to this second strategy. The balanced Christian avoids both extremes: they are aware of him and his tactics (in fact, we are told to be so5), but their focus is on Jesus and the almighty power of God. For that reason, I won't spend a lot of time on this topic, but I feel it necessary to make a few quick points.
Satan is a being created by God (as are all beings except for God Himself), who was cast out of the Presence of God along with one third of the angels who followed his attempt to elevate himself to the level of God6. The lament over the King of Tyre is considered by many to have two meanings: one of which is a description of Satan and his folly.7 I'm sure that even the lowliest angel has an appearance that would blow the mind of any human that saw their actual form. But none of them have anything on Satan. So get rid of the idea of a red suit and a pitchfork. The enemy appears in a very appealing form, usually as servants of God8. Maybe a red suit and pitchfork is an appealing form to some people - the enemy will happily oblige whatever form will most likely dupe us and draw us away from God. After all, who would follow after him if he were unappealing? Whatever you are inclined to believe is how he will appear. Maybe it will be as extraterrestrials, or a great spiritual/religious movement, or a "nature spirit", or an angel. Religion has always been a tool of the devil - even pseudo-Christian religion. I'm sure the enemy gets a laugh when they can get someone following them while they think that they are following God. There are countless ways they will try to deceive us - but it will always be something that our flesh is inclined toward. And the devil is always looking for ways to divert people away from a loving God.
It is for this reason that people without the Holy Spirit are deceived into following the Anti Christ as described in the book of Revelation. They won't view him as the Anti Christ - they'll see him as a great savior and/or political leader and they will blindly follow after him. Consider how most Germans followed Hitler at the start. It wasn't until later that they began to fear the SS. It was only in the aftermath that they learned about the concentration camps. Now, imagine most of the whole world following after this charismatic leader believing that they are in the right - like mice feasting on the cheese before the trap snaps shut on them.
Along with the strategy of pretending that he doesn't exist, he gets many people to dismiss the idea of demon possession, or demons themselves. The fact is, demons can possess people9. I don't know under what circumstances it happens, and I suspect it is relatively rare - certainly much rarer than some people seem to believe. However, I do not believe that believers can be possessed. A demon cannot live inside someone in whom the Holy Spirit of Almighty God is dwelling. Believers can sometimes be "oppressed" by demons, but that is a far different thing. And it is rare and temporary for one simple reason: believers have authority over demons and rebuking them in the name of Jesus is enough to send them running10. Although I know that such authority allows believers to drive demons from those who are possessed, people who go around making a "living" doing that have fallen prey to the second basic strategy of the devil. This is something that should happen under the direction of the Holy Spirit during the normal course of living out a holy life. And sometimes it is hard to discern if someone's behavior is due to emotional/mental/physical illness, a desire for attention, drugs, or just plan sinfulness. Some people, having seen or read scenes from various media, simply act like they are possessed. I think that it almost always takes the Holy Spirit to point out to us if there actually is a possession. Even Paul didn't recognize a possession for several days while he was preaching - or he recognized it but wasn't led by the Spirit to do anything about it immediately11.
What powers does the devil have at his command? The Bible doesn't go into great detail, and all other sources of information are suspect. I believe one reason the Bible doesn't go into detail is because it is unimportant and would simply provide a way for some people to obsess over the topic (see strategy 2). There are a few points we can take from scripture, and things to look out for. First, the angels, as spiritual beings, are immortal and have been around for thousands of years. Even if they were only as intelligent as us (and I suspect they are smarter than the smartest of us), their experience makes them more "street smart" than any of us humans. They know the Bible better than any of us. They have seen God's Face and - excluding communion with Him - they know God better than we do. Being in the spiritual realm, they see reality more clearly than we can. We don't know the full extent of their powers, but they are not omnipotent - only God is. But their main power appears to be in their ability to deceive mankind12. They hate and fear Almighty God and are hell-bent on preventing as many people as possible from being reconciled with Him - and to keep those who have been reconciled from being effective in the Kingdom of Heaven. We have nothing they need, and so the idea of making a deal with the devil is ludicrous. If the enemy can deceive someone by supposedly making a deal with them, they will do so. But they cannot be trusted. Faustian bargains are the imaginative creation of human conceit. They have no need of human souls - they only want to deny those souls to God. They cannot be controlled by "magic" of any kind. Nor can non-believers rely on the power of God to drive out demons (and such attempts can end badly)13. The only thing they fear is more powerful spiritual beings. It is only this terror of Almighty God that keeps them in check in this world. At some point, God will remove His hand and give them full reign for a very short time, coinciding with the Anti Christ. I can only say I'm glad I won't be around to see the kind of hell on earth that will be.
One last thing I'd like to point out. I've noticed a segment of believers who blame demons for what they have done. I inwardly cringe when they blame a "demon of lust", or a "demon of greed", or a "demon of anger", or say "the devil made me do it". Yes, the devil, the world, and some people, may tempt us. But when a believer sins it is because they gave into the temptations of the world, the flesh, or the devil. Placing the blame on a demon, or the way someone treated you, or the way a woman is dressed, or whatever, is simply a matter of being unrepentant. And if you don't accept the fault, you cannot accept the forgiveness14. Stop using the devil as a scapegoat and just confess that you sinned, that it is all on you, and repent.
And that is all I will say directly about the devil for now. Again, the believer has nothing to fear from the devil - what is more to be feared is becoming obsessed with the subject and taking our eyes off of Jesus.
1 Matthew 4:1-11, Mark 1:12-13, Luke 4:1-13
2 Hebrews 4:15
3 Job 1:6-12, Matthew 4:8-10, John 8:44, 14:30, Ephesians 6:11-12, Revelation 12:9
4 Job 2:1-6
5 2 Corinthians 2:10-11
6 Revelation 12:3-9
7 Ezekiel 28:11-17
8 2 Corinthians 11:14
9 Matthew 12:22, 17:18, Mark 5:1-13, 7:29, Luke 4:33-35, etc.
10 Matthew 10:1, Luke 9:1
11 Acts 16:16-18
12 John 8:44, Acts 5:2, 2 Corinthians 11:3-14, 2 Thessalonians 2:9-10, etc.
13 Acts 19:14-16
14 1 John 1:9