A sorcerer who enslaves and sends a low-ranking demon or human spirit to another human is said to have a familiar spirit. Because this spirit is a sorcerer's slave, it is sometimes referred to as a household servant. These spirits are able to remain in the victim's body because of his weaknesses and sins. As the familiar spirit continues to stay in a person's body, it learns all his weaknesses and secret sins. The longer a familiar spirit stays in a body, the stronger the hold he gets. Due to years of weaknesses and sins, these spirits gain significant hold over a human and hence it's not easy to overcome a familiar spirit.
Complete deliverance requires a deep commitment from the victim as it require uprooting of thousands of deep roots. Even if a familiar spirit may leave someone for a while, it can return, if the person falls back into the secret sin. Those who love God benefit from this spirit because it helps them overcome their shortcomings and get established in holiness.
“‘And the person who turns to mediums and familiar spirits, to prostitute himself with them, I will set My face against that person and cut him off from his people. Consecrate yourselves therefore, and be holy, for I am the Lord your God. And you shall keep My statutes, and perform them: I am the Lord who sanctifies you. Leviticus 20:6-8
“A man or a woman who is a medium, or who has familiar spirits, shall surely be put to death; they shall stone them with stones. Their blood shall be upon them.” Leviticus 20: 27
I experienced an illness for about a year when I was under the influence of a familiar spirit. It took me several months to get rid of the illness and the spirit, as I dealt with various weaknesses.
I have also guided a few people to overcome their familiar spirits.
What are familiar spirits and how you can be delivered from them