Other gods exist.
Read Genesis 3; Exodus 15; Deuteronomy 32; Judges 11:24; Psalm 82, 95, 97; 1 Corinthians 8:4-6, 10:18-22. Here’s the bottom line: the gods (elohim) of the nations are actual spiritual beings that are opposed to the one true God—Yahweh Elohim. These false gods are not non-existent or make-believe. They are very real and very powerful. In the Gospels these beings are sometimes referred to as “demons”—the background of which comes from the Greek translation of Deuteronomy 32:17, “They sacrificed to demons, not God, to gods they had not known; to new gods who had recently come along, gods your ancestors had not known about.” Understanding that other gods exist is crucial in order to grasp the reality of our spiritual battle. The Gospel is that God through Christ is victorious over these gods and their power. As Paul says, “This power he exercised in Christ when he raised him from the dead and seated him at his right hand in the heavenly realms far above every rule and authority and power and dominion and every name that is named, not only in this age but also in the one to come.” (Eph. 1:20-21) Yahweh through Jesus and the Spirit is reclaiming humanity—a humanity that left to our own devices joins with these false gods. Praise Him that in the end, God will be victorious and His people will shout again, “Who is like Yahweh among the gods!?” (Exodus 15:11)
“Demon-possessed” is a misleading term.
Most English Bible translations translate the Greek verb daimonizomai as “demon-possessed.” This isn’t a wrong translation as much as it is a misleading translation. The word “possess” is not in the Greek text. It’s supplied to make sense of the word. The issue is that “possession” can be taken to denote ownership, as if the human is now owned by the demon/Satan. However, the Greek term does not denote ownership. Rather daimonizomai denotes inhabiting influence. I prefer the translation “demonized” as it implies different levels of daimonizomai. This has ramifications for whether a believer can be “demonized” or not.
Remember that the enemy is real, but focus on Christ.
There’s two mistakes that we should be sure not to make when thinking about spiritual warfare. First, not thinking about it at all. This is where most of us find ourselves. We are all constantly breathing in the foul air of naturalism, so supernatural beings are rarely if ever considered in our daily lives. But, if you spend a few seconds in the Bible, you will see that this is not the Biblical worldview. Indeed, many people who make this mistake are actually very involved in spiritual warfare whether they know it or not. Nonetheless, the second mistake we need to be wary of is being overly-focused on the enemy. Some Christians rightly understand the importance of spiritual warfare, but are so focused on the enemy that they forget to gaze upon the victorious Christ. Jesus has defeated, is defeating, and will defeat the powers of evil. Christ is our focus.