Warning, Daemonic Possession is Very Real; a rational argument.
It is said that we are what we eat. The truth of this statement becomes undeniable if stated as: we are greatly influenced by what we consume. Whether it is consumed through formal education or through entertainment or culture, it all adds up to affect our decisions, choices, actions, our self.
Without getting in the whole nature vs. nurture, we can settle on the fact that, at least to some degree, what we take in becomes a driving even controlling force in the make up of our self. So if what we take in can control us, should we not choose to take in that which is good over that which is evil? Fore if we take in evil than we allow ourselves to be possessed by evil, even if only in degrees.