Master Qingliang Yue: Some people are afraid of being possessed by the Devil. One thing we must know is that we are all currently devils. There are only two states of mind: fundamental and derivative. We and devils all live in the derivative state of mind, so we are no different from devils. Unless we return to the state of Rulai Zang (Tathāgatagarbha), we could never avoid being possessed. We simply move from one devilish state to another. There is not such a thing that we move from the state of Buddha into the state of devils.
So, without proper methods, we only move from one devilish state into another. We consider this state as devilish simply because it is different from a state which is commonly possessed by ordinary people and so regarded as healthy. For example, sleep is a disease that we all have. We all take for granted that we need to sleep. Those who do not sleep are considered to be sick. Another example. We are in logical thinking all the time. Those who could kill their thoughts are thought to be unusual. Therefore, do not worry about being possessed. We are actually living in different devilish states.