By St. Theophan the Recluse
“My name is Legion, because we are many” (Mark 5:9). Spirits are not corporeal, so they do not fill or occupy places like bodies. This explains the physical possibility of many spirits staying in one person. The possibility of morality on the part of spirits is clear from their immorality or the absence of any moral principles, and from the human side – from the multilateral contact of his spiritual structure to the gloomy realm of impure forces.
But this only explains the possibility; the reality of the possession of demons is subject to conditions that we cannot determine. We can only say that the infestation of spirits is not always visible, it is not always revealed by the known actions of the possessed. There is an infiltration of spirits that is undetectable, hidden; there is also the power of the spirits over the minds, besides the body, when they lead them as they please, through the passions at work in them; people think that they are all acting on their own, being a laughing stock of evil forces. What to do? Be a true Christian, and no enemy force will defeat you.
But this only explains the possibility; the reality of the possession of demons is subject to conditions that we cannot determine. We can only say that the infestation of spirits is not always visible, it is not always revealed by the known actions of the possessed. There is an infiltration of spirits that is undetectable, hidden; there is also the power of the spirits over the minds, besides the body, when they lead them as they please, through the passions at work in them; people think that they are all acting on their own, being a laughing stock of evil forces. What to do? Be a true Christian, and no enemy force will defeat you.