A demon as I have previously explained are either the souls (intelligence) of someone that died by suicide or they are the souls of people that were sacrificed in a ritual and their blood combined with the parts of an animal. Therefore, the demon is the soul of a human being that cannot re-incarnate, due to the fact that they died by suicide or ritual sacrifice.
The powerful demons out there were the ones, when a human being was sacrificed in a ritual and the blood is combined with the ashes of a snake, or tiger, or lion.. This combination forms a demon soul, that has the skin of a snake or tiger or lion, depending on what animal was used in the ritual.
For a demon to possess someone for the first time, the blood or sperm of someone that has once been possessed by a demon has to enter their body. Example, if a person who gets possessed by a holy ghost in a church were to add some of their blood to any juice and a person drinks, Anyone that drinks that juice from then on will be possessed by demons. All those that drunk that juice wherever they go, if there is a so called holy ghost in the place, they will be possessed.
The other common means is through sexual intercourse with someone that has once been possessed by a demon. The sperm of a human being is a blood cell, so if a person engages in unprotected sexual intercourse with someone who has been possessed by a demon, that person will automatically be possessed in the future by all kinds of demons.
Now it takes a stronger demon to cast out a weaker demon. The philosophy here is: iron cuts iron. For a person to cast out a demon, that person himself must have a stronger demon. This is where the different animals used in the ritual comes in. For the stronger animals will overpower the weaker animals.
Beware that being possessed by a holy ghost does not make a person a saint or an angel. The reason is, a holy ghost is not a spirit of God, but rather it is a human soul. Demons are not spirits, they are human souls. Hence, even though one believes they have been born again by a holy ghost, they are still human, subject to all the diseases of any human being, and they will die and re-incarnate like any normal human being. People that are transformed into angels, have a blood type of angels, which makes them immune to diseases. These are eventually transported to heaven by a spirit.