Along with the God-given protections given us at birth from the Demonic here on earth, 
these physical bodies are ours and belong to us. God has given us the exclusive right to these bodies as they are an integral part of the plan of salvation. The demonic entity can’t take possession of them, in other words inhabit them unless we lose control of them, or give explicit permission for the demonic entity to inhabit them. This can occur in two ways.
First is losing control of our bodies. This is something we can do voluntarily through excessive drug and alcohol abuse. A prolonged state of voluntary altered consciousness allows the demonic to access a chink in the spiritual armor which is provided by God at birth and can give them access to inhabit and take control of our physical bodies. Remember they were denied physical bodies when they chose to rebel against God. Possession is the only way they can experience having a physical body. It is also the ultimate insult to God in their minds to take possession of bodies that are a gift from God to us and do not belong to them.
The second is willfully granting permission. The willful granting of permission comes in two forms. The first is what is called perfect possession. This is when someone, usually a cultist, or someone practicing dark magic knowingly and willfully grants demonic spirits access to their bodies. The demonic hop on in as co-pilots usually for life. They can sit quietly and watch while the person goes about their day, or they can exert full control over the body. The possessed will either watch the goings-on of their body while they have no control or they can simply blackout and not remember the things done while not in control of their bodies.
Those perfectly possessed can lead a full life, full of destruction, blasphemous acts, and ruination, or the demons will eventually use the body as a toy eventually destroying it before abandoning the ruined body to death.
The second form of willful permission is the most dangerous and that is part of the oppression process. After a person has been so physically and psychologically drained through the oppression process the demonic essentially promise the person the torment will stop if they just surrender. The worn-out target then surrenders to the demonic thereby granting permission for possession. These possessions can be relatively short-lived as the demonic entity will treat the newly obtained body the way a teenage hoodlum will treat a freshly stolen expensive sports car. In a short time, the body will be wrecked through hard living. The demonic want to destroy us so they will do whatever they can to wreck and destroy the body they have taken possession of.
It is rare for a single demonic spirit to inhabit a possessed body. Usually, a possession consists of several if not dozens to hundreds of entities. Possessions are not a common occurrence and every demonic spirit wants a turn at the proverbial wheel.
During possession, the possessed can often speak backwards or in languages the possessed person should have no knowledge of. The possessed often speak in Latin and will spout foul blasphemies against God and spew all manner of filth and profanities.
The possessed will speak in voices that are not their own. Usually, they are deep guttural voices. The physical features of a possessed person can literally change. A possessed person can literally age before one’s eyes. The old saying that eye are the window to the soul takes on a new meaning with the possessed as their eyes also change.
The possessed will speak of things that will occur in the future. These things are generally accurate. They will also speak of things the possessed person should have no knowledge of.
The end goal and final stage of possession from the demonic standpoint is death.