It’s an old tension or conundrum, but still one worth pondering. On the one hand, it’s a mistake for the Christian to give Satan too much credit for what is going on. Some Christians find demons under every rock in the NT, and even talk about a demon giving them a cold etc. Of this sort of demonizing of everything that goes wrong in life, the NT shows no hint.
Indeed, it is worth noticing that Paul never even uses the word demon, save once (‘you cannot share in the table of demons and also the table of the Lord’). If Christ is the Lord of your life, Christian do not need to fear being possessed by demons. Pestered perhaps, bother or bewildered from the outside or even persecuted, pressured, and harmed perhaps, but not possessed, not spiritually endangered.
If you read Rev. 2-3 carefully you will notice that the powers of darkness are said to be able to harm believers physically, but not spiritually. They are protected spiritually. Greater is he who is within you than the forces that are in this world. Notice in Job 1-2 Satan is able to harm the body, but not the spirit of Job. That’s on the one hand.
On the other hand, we have plenty of evidence that Satan still wreaks havoc in the world. Jesus for instance talks about Satan. In Mark 4.15 it is Satan who comes and takes away the Word from the lives of those who’s hearts are like a well-trod path. In Lk. 22.31 Satan is said to be allowed to sift the disciples but Jesus has prayed for Peter so that his faith does not fail altogether, and when he repents and turns back to Jesus he is supposed to strength his fellow disciples.
In Ephes. 6.10-18 Paul is quite frank that we are in a struggle with the Evil One, who has fiery arrows and we must struggle to stand against the onslaught of the evil one in this evil age. In John 8.44 Satan is seen not merely as a tempter but as a liar and a deceiver, and the father of lies. Acts 26.18 says that pagans are under the power of Satan and need to be rescued, and indeed Lk. 10.18 makes clear that exorcisms were one form that rescue took in Jesus’ ministry. He was able to bind the Strong Man, and set his captives free. But the battle is definitely not over. Revelation records a threefold fall of Satan— from heaven to earth, from earth to the Pit, and from the Pit to the lake of fire. According to that book, Satan is alive and well on planet earth these days, and according to 2 Cor. 11.44 he has many disguises, even appearing as an angel of light. 2 Cor. 12.7 says that even Paul was harmed by a stake in the flesh courtesy of Satan. In 1 Thess. 2.18 Paul freely admits that while he wanted to come visit his converts, Satan prevented him from doing so.
On the other hand, 1 John 2.13-14 says Satan can be overcome by believers, and that he flees when he is resisted if one turns to God. ( James 4.7).
C.S. Lewis once said it is perhaps the greatest trick or smoke screen or deception of Satan to convince people that they are too wise to believe in him. But at the same time, it is a mistake to give him too much credit as well. There is a balance between the extremes reflected in the NT, and this is in part because all of the writers of the NT believe they live in ‘this present evil age’ which, now that the Kingdom is breaking in, is passing away. Satan, after the death and resurrection of Jesus is fighting a rear guard action, for he has already lost the battle of D Day, and V-E Day is coming when Christ returns.
The believer then lives betwixt and between, overcoming evil with good, but not surprised or caught napping when temptation happens, wickedness has it’s day, believers suffer, and all is often not right with the world. The good news is— Christ has overcome the world, through faithful life and death and resurrection, and that is our recipe for overcoming as well.