Psychiatry is still an infant and imprecise science, more about description and association than causes and cures. So saying, “Well, some parients do and others don't, we don't know why,” isn't simpler than saying, “In some instances malevolent spiritual beings may be involved.”
Now, case study:
Very intelligent (honors scholar type,) successful, kind lady. Very sweet and considerate of others. She is diagnosed as bipolar but it is controlled enough by meds that you would have to know her for a while to discern it.
She WILL not go to church, even for a academic lecture. She won't listen to excellent music if it's church-related. Holst's Short Festival Te Deum? No way.
She insists that no one discuss theology or philosophy in her presence. Then she makes little snide remarks about Xtian thought!
A meticulous researcher, she succumbs to all the popular errors about Catholic thought!
She says she is going to hell. She smiles, but she says it.
She is unbaptized. She doesn't have even the most common protection against demons.
So: a lovely, insightful, intelligent, generous person who becomes startlingly snarky, unfair, unscholarly, and almost phobic about church related things.
There are more observations, but this is enough to make demonic oppression (NOT possession) something which ought to be considered. The specificity, strength, and nastiness of her aversion is remarkable.
It is often very difficult to determine if the cause is mental illness, possession, or both. That is why prayer for the person is always the safest and preferred action.
Interesting anecdote (no sarcasm intended).
I'm not saying that one should a priori discount possible spiritual involvement, but it would be ill-advised to begin looking there.
Regards,