It is possible for believers as well as unbelievers to be DemonOpressed.
In an unbeliever, the demon may or may not directly torment the victim.
In an believer, demonic oppression always manifests torment. Any believer who has a Strong Hold in their life is Demon Oppressed.
Unlike Demon Possessed people, DemonObsessed people can typically function normally most of the time. They can 'manage' the demon.
Demons are always up to no good. They are like their master, Satan, who steals, kills and destroys.
In order for a person, believer or unbeliever, to be delivered from the Strong Hold of the demon, they must repent. They must choose to get rid of the Strong Hold. When they choose to be free, they can be on the authority of Christ Jesus.