Answer
Since demons do not really exist, you can never really be possessed by demons. However, you can be possessed by superstious belief - that is your choice. And superstition can make you believe you are possessed by demons. Since there are no fixed rules for superstition, you have to ask someone who is equally superstitious. If you are not happy with that person's answer, you can ask another superstitious person, who could give you a different answer.