I've seen evil spirit possess a person. Its the sickest thing to see.
I can also assume thousands of people are in mental institutions and such places for the reason that they are actually possessed.
It is so easy for person who believes only in things proven by science to consider person possessed as schizophrenic or that they have split personality.
I don't like to talk about the whole possession in much detail but I would like to bring up one detail since it might one day help someone else.
Person possessed was Wiccan by religion and this person had the star shape pentagram on a necklace. (the Wiccan symbol pentagram with 2 points down one up)
This spirit possessing tried to leave in this body he was possessing now. I held the body down and other people made effort to get rid of this spirit. They tried to make holy water which didn't seem to work, they brought a cross but this spirit only laughed at it.
After struggling to keep the spirit from leaving with the body he was possessing I noticed that the hand of the body touched the pentagram star and the reaction gave it all away.
It looked as it burned and the spirit violently pulled the hand away from the pentagram star. After seeing this I took the pentagram and pushed it against the chest of the body he was possessing and told this spirit I would hold the pentagram there forever if he didn't leave. When I pushed the pentagram down this spirit screamed as it would have been burned with hot iron. The screaming only lasted for few seconds and then this spirit could not take the pain the pentagram obviously caused and it left. This happened by the body fainting and after few seconds woke up with the original owner of this body crying and telling me how this owner was actually out of the body while the spirit was inside and saw everything happen just few feet away and was so scared to maybe never be able to come back to the body.
So if some conclusion needs to made from this I would say that pentagram seems to be the strongest item to use against possessing spirit and getting them out.
I'm not a Wiccan or believe all the things like the magic and such they practice but I do wear a pentagram on my skin at all times after what I saw.