This question is too important to mess up, so we'll take a few posts to get it right. In this first one, we want to get a bigger picture and ask, "How does the Bible describe demonic influence?"
Satan and demons appear in the Old and New Testament. We’re not concerned here with all the Bible teaches, but instead want to focus on how demons interact with humans. Most of the references are in the New Testament, with quite a few in the Old. They can be placed into four different categories:
inflicting physical illness
ideological
influencing the larger world order, and
possession or control of individuals.
Before jumping to the last one, let's look at the three to gain some context.
Physical affliction
This first one might surprise us, but it was something Jesus encountered. For example, “they brought him a demon-possessed man who was blind and mute, and Jesus healed him, so that he could both talk and see” (Matt.12:22). And again, “Lord, have mercy on my son,” he said. “He has seizures (selēniazomai) and is suffering greatly.
He often falls into the fire or into the water” (Matt.17:14-20). Jesus encountered a man who could not talk (Matt.9:32) and a woman who had been crippled for eighteen years (Lk.13:11), both of which are attributed to the work of evil spirits. In the Old Testament, we’ll recall that Satan “afflicted Job with painful sores from the soles of his feet to the crown of his head” (Job 2:7).
Ideological persuasion
The second is by far the most prominent. It deals with influencing people not physically, but by inserting wrong ideas or thoughts. Influencing the mind, but not controlling it. This accounts for almost a third of the references to demonic influence. We read about it in the first pages (Gen.3:1), in the last pages (Rev.16:14), and in between (2 Cor.11:3-4).
It can be indirect, such as through false prophets, but at other times it isn’t as clear.
“Dear friends, do not believe every spirit, but test the spirits to see whether they are from God, because many false prophets have gone out into the world” (1Jn.4:1).
“The Spirit clearly says that in later times some will abandon the faith and follow deceiving spirits and things taught by demons” (1Tim.4:1).
“See to it that no one takes you captive through hollow and deceptive philosophy, which depends on human tradition and the elemental spiritual forces of this world rather than on Christ” (Col. 2:8).
“Be alert and of sober mind. Your enemy the devil prowls around like a roaring lion looking for someone to devour” (1Pe.5:8). See also James 4:7; Lk.22:3, 31; Jn.8:44.
Regardless of the form, Scripture is clear that this is the most common form of demonic interaction with humans.
Influence over the world/world order
The third is a broader influence of the evil realm over the world. Paul says: “For our struggle is not against flesh and blood, but against the rulers, against the authorities, against the powers of this dark world and against the spiritual forces of evil in the heavenly realms” (Eph.6:12). The influence is often hidden from view until John details it in his final book. “They are demonic spirits that perform signs, and they go out to the kings of the whole world, to gather them for the battle on the great day of God Almighty” (Rev.6:14; 12:9; 20:3,10; cf. Dan.10:13). There is an invisible world, apparently hierarchical in nature, ruled by Satan that is active in working out plans and schemes from a historical point of view. These spiritual powers contribute to the course of human history.
And finally, the last category is what is commonly called “possession.” Let’s note right off the bat that the Bible never refers to demon activity in terms of “possession.” But we'll tackle that in part two, when we ask the question, "What does it mean to be 'demon possessed?"