I trust that we all recognize this as the response from Jesus to Peter when Peter told Jesus that He would not die on the cross. Now we know what happened here. Satan convinced Peter that the messiah would not die but live to rule the world. Peter was not knowingly following Satan. He was deceived.
Paul also speaks of those whom the Devil has deceived. "With gentleness correcting those who are in opposition, if perhaps God may grant them repentance leading to the knowledge of the truth, and they may come to their senses and escape from the snare of the devil, having been held captive by him to do his will." (2 Timothy 2:25-26) It's funny how we always think it's the other guy who is influenced, not us. Why, we would never listen to the Devil. But people don't necessarily consciously yield to the Devil. They are usually unaware that the Devil had anything to do with it.
Paul said something else important about the Devil. He said, "We are not ignorant of his devices." (2 Corinthians 2:11) I think that Christians today are ignorant of Satan's devices, at least some of those devices. How many Christians understand that the Devil has something to do with things like sickness or mental illness?
"You know of Jesus of Nazareth, how God anointed Him with the Holy Spirit and with power, and how He went about doing good and healing all who were oppressed by the devil, for God was with Him." (Acts 10:38) Everyone whom Jesus healed was oppressed by Satan. Satan had something to do with their condition.
When Jesus healed people, He sometimes cast an evil spirit out of them. " When evening came, they brought to Him many who were demon-possessed; and He cast out the spirits with a word, and healed all who were ill." (Matthew 8:16) The expression "demon-possessed" is not a really good translation. The word 'possessed' is simply not present. The term 'demonized' is more accurate, yet this term means something else in English. To 'demonize' someone means, today, to slander someone as evil. So I think that the term 'oppressed', as in the ESV and in Acts 10:38, is better. A truly demon-possessed person would be like the man who had the legion of demons who were sent into the pigs.
So I think we might want to retire the term 'demon-possessed'. It conjures up all sorts of strange images. Jesus was healing people and while He was healing them He discerned that some had evil spirits oppressing their minds and bodies. I am assuming, of course, that some were mentally ill while most were physically ill. (The NT does not make the modern distinction between the two.) So, along with Acts we have some who are ill, physically or mentally, who have evil spirits oppressing them directly, but Acts clearly shows that all sickness is Satanic oppression. He is behind all of it.
Some people think that the only thing that Satan does is 'possess' people or deceive them or tempt them. But it goes beyond that. Evil spirits seek expression in humans, in their minds and their bodies. They have no physical bodies by which they can express themselves so they try to use us.
I really believe that the Devil not only deceives and tempts us to sin, but that when he succeeds that he can make inroads into our lives. He uses our natural weaknesses against us to gain a foothold. Eventually, demons can oppress us and take us further into bondage as we yield to them. I do not think that alcoholism is a physical illness. I think it's a demon that gets hold of some and puts them in bondage. The same is true of various kinds of addictions. It can be pornography or some other sort of sexual sin. The presence of a demon makes it very difficult to break free.
Now I hope that nobody has jumped to the unjustified conclusion that I think that alcoholics or people with depression are "demon-possessed", whatever that may mean. I have ministered to some with depression and have encouraged them to resist the Devil whenever they get symptoms of depression. I realize that many must take medicine to counter some chemical imbalance that may cause depression. But they must understand that the Devil will try to take advantage of that weakness to bring about an episode of depression. We have to keep our minds continually renewed to the Word so that our thinking does not contribute to depressive tendencies.
Another area of human weakness that the Devil exploits is in the area of sex. The sexual instinct in us is so strong that it is easy for Satan to lead us into bondage. The Bible talks about at least some evil spirits as "unclean spirits". Some of that uncleanness is clearly sexual uncleanness. "Now the works of the flesh are manifest, which are these; Adultery, fornication, uncleanness..." (Galatians 5:19) Uncleanness is mentioned alongside adultery and fornication. This is sexual uncleanness like those sex sins mentioned in Leviticus. An unclean spirit can lead people into unclean sexual activities that hold them in bondage. We call these things 'addictions' but they are really spirits holding people in bondage.
Were all the people that Jesus cast the Devil out of "possessed" by some demon? Or were they oppressed by him, perhaps having unwittingly yielded to him. I think it's the latter. There was a young guy in a church I attended some time ago. He gave his heart to the Lord, but soon after got back into drugs. (He thought that he was hiding it from the rest of us in church.) A minister prayed for him at the altar and rebuked the demon that was holding him in bondage. He not only was set free, but he eventually became a pastor. Before that minister did that, I am sure that you would not have convinced him that a demon was oppressing him.
It's important that we don't go extremes in this area and see demons everywhere and in everyone. Yet, at the same time, we do not want to be ignorant of Satan's devices. He seeks whom he may devour. We have all yielded to the Devil at one time or another. We have to learn to resist him. "Resist him, and he will flee from you." We can resist the Devil with God's Word spoken in faith. The best thing, though, is just not yield to him. Just say, no!