The word familiar is from the Latin familiaris, meaning a "household servant," and is intended to express the idea that sorcerers had spirits as their servants ready to obey their commands. Those attempting to contact the dead, even to this day, usually have some sort of spirit guide who communicates with them. These are familiar spirits.
Leviticus 19:31; 20:6, 27; and Deuteronomy 18:9-14 refer to “mediums and familiar spirits” and forbids being involved with them, as they are an abomination to the Lord. A medium is one who acts as a liaison to supposedly contact or communicate with the dead on behalf of the living. In reality mediums are contacting demons who convince the mediums that they are “familiar” and can be trusted and believed. The practices associated with mediums and familiar spirits were banned in Israel, and the punishment for practicing such things was death.
Familiar spirits and spirit guides are under the control of their master, Satan. They influence people to spread lies and deceit in order to thwart the kingdom of God. To knowingly open oneself to the work of demons is an evil thing: "Let no one be found among you who sacrifices his son or daughter in [a] fire, who practices divination or sorcery, interprets omens, engages in witchcraft, or casts spells, or who is a medium or spiritist or who consults the dead. Anyone who does these things is detestable to the LORD" (Deuteronomy 18:10-12a).
Some avenues through which demons or "familiar spirits" can gain entrance into a person's life are divination, transcendental meditation, visualization, necromancy, witchcraft, drugs, and alcohol. These are all activities that believers are exhorted to avoid. Instead, we are to be filled with the Holy Spirit, with love, with joy, and with the fullness of life that comes from Jesus Christ. We are also to be on guard, "for our struggle is not against flesh and blood, but against the rulers, against the authorities, against the powers of this dark world and against the spiritual forces of evil in the heavenly realms" (Ephesians 6:12).