Do evil spirits exist or is just people's superstition or imagination? Whether you believe that they exist might depend as much on your understanding of the Bible as on the environments in which you have lived.
Both Paul and Jesus came in contact with what the New Testament describes as “unclean spirits”, “evil spirits” or “demons”. Nowhere does the New Testament use the word “possessed” even though some translations use that word. The usual phrases are that a person “had an unclean/evil spirit” or that they were “demonized”. The reason is that except for cases like the Gadarene demoniac, the other examples relate to the affect a demon has on an aspect of a persons's life, eg that they are dumb or deaf, not the control of their whole life.
One question is whether Christians can be affected by evil spirits. Paul certainly thought that evil spiritual and demonic powers were strongly at work against Christians. In Ephesians (4.27) he warns the believers not to let the devil get a foothold. That is by lying, stealing, and getting angry. In Colossians (2) he thinks that believers can come under the power of the evil powers by using systems of religious rule keeping to help them become more godly.
The background to this is what Paul called “the ruler of the kingdom of the air, the spirit who is now at work in those who are disobedient.” We live in a world where the evil one holds great power.
The Bible does not encourage us to take the fight directly to the demonic powers. Our commission is to help set people free by means of the gospel. For ourselves when we face spiritual attack is to stand firm. Stand against the devil's schemes. So we can keep on with the main game of making disciples.
But the devil's schemes are often subtle. They often have a lot to do with deception and lying. They often make us feel comfortable about what is wrong and uncomfortable about things that don't matter. The devil's schemes in western culture seem to be about providing sedatives, distractions and catering to our pleasure seeking, rather than any threats of violence or material loss.
And the protection? The Lord's mighty power through the gospel.
Dale