Peck also discussed the question of the devil.[8] Initially he believed, as with "99% of psychiatrists and the majority of clergy" (Peck, 1983/1988,[8] p182), that the devil did not exist; but, after starting to believe in the reality of human evil, he then began to contemplate the reality of spiritual evil. Eventually, after having been referred several possible cases of possession and being involved in two exorcisms, he was converted to a belief in the existence of Satan. Peck considered people who are possessed as being victims of evil, but of not being evil themselves. Peck, however, considered possession to be rare, and human evil common. He did believe there was some relationship between Satan and human evil, but was unsure of its exact nature.
This excerpt is from M. Scott Peck’s Wikipedia page. I highly recommend reading the entire section on Evil.
https://en.m.wikipedia.org/wiki/M._Scott_Peck
I read it, I still don't know what to think. I know there is evil everywhere, what I don't know about is if there is a creature called the devil, I think that's silly, especially for a psychologist to claim he saw. With all the billions of people on earth, his likeliehood of running across a couple that were possessed by satan himself seem remote.
I would way more easily believe that there are just plain evil/dark/life denying energies and individual spirits who roam about and influence us, I'd believe that way before I'd think Peck met the devil.
I have witnessed "possession" when I lived with the preacher. I later came to believe that it was the girl regressing into a traumatic molestation scene which was trapped in her mind, not some demons, but I still cannot explain how the girl knew things like "there are more records in the house" (they burned all secular records and then they dug out mine and burned them all, how did she know that I had a collections? She didn't live there at first) Also, there was the pungent smell of sulfur which is always associated with hell. When the pastor suggested I might be possessed, I kinda freaked because I didn't want anyone praying over me like they did this girl, however, on a couple occasions I was many, many miles away and I could smell that same sulfur smell, it really effected me. I still don't know what to make of it all and yes, I do believe that evil spirits influence us, it's the only way I can make sense of the monstrous stuff human beings do.
I also think we have to look at other answers first, as in this girls case, I really do think she was going into a trance/regression state of being molested, just the way she was moving like someone was having sex with her. Either way, it was unsettling to say the least and when asked how many demons were in her she replied "legions" and "666". I really wish I could ask this woman today what she thinks of it all in retrospect.