That which we know not. We would love to eliminate the words Satan and Demons from our vocabulary, but then they would be in our memories. Oppression pertains to forces or spirits from without that attack the person's mind with confusion and depression. If expellation is not exercised, usually the spirits will cause great misery and ultimately death to come upon its victim. Even to the trained eye, it is almost impossible to know if a person is SOP or just mentally ill.
Symptoms resemble mental illness.
Likewise, anyone possessed to a great degree will exhibit like symptoms. As they went out, behold, they brought to him a dumb man possessed with a devil. Doctors may look at this account and possibly argue that due to some form of trauma, he refuses to speak. After all, it is nearly impossible to make distinctions between the possessed and the mentally ill. Experience in working with both is an advantage.