I am not even convinced that Satan was a physical or spiritual being, but "The Accuser" or the "the Tempter". Which may mean that when Satan "enters" someone, they are giving into temptation to do something not approved of. Which may also explain why Jesus said to Peter "get thou behind me (go away), Satan".
All flesh is subject to temptation, and it isn't always possible for us to withstand it.
All flesh is subject to temptation, and it isn't always possible for us to withstand it.