First, to go back to "mental illness" and away from the demonic, at least for the moment.
I am familiar with friends, siblings, I would say have PTSD, although I saw one counselor say that in the case of Christians the D for disorder should be dropped, as the Spirit who gives us a sound mind (2 Tim1:7KJV) indwells them.
Re the siblings, girls, then below the age of 10, one suffered sexual abuse by a person close to the family for a relatively long period. There is such a thing as having the psyche traumatized and "coping mechanisms" adopted "unconsciously" so as to be able to function, such as forgetfulness of the trauma, and other psychological phenomena, eventually, in adulthood, manifesting as distinct post-trauma stress patterns and behaviors. In the abused one, it was severe, and – even after the trauma was remembered – the patterns and behaviors were not controllable, or even, mostly, discerned. In the other sib, having the relationship disrupted by a third party caused a differing, and still very severe PTSD.
In the abused one, she became a genuine Christian, and this in itself causing a major stabilizing force in her life – Jesus, and continual access to Him and His guidance and care. She is a godly woman in her mature age, yet still has severe blind spots and disability. Mental illness, as noted above, can be caused by psychological damage.
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Re the demonic: a factor not often realized in these days due to its wide-spread use and cultural spin of benignity, is sorcery, in the NT and OT Greek Septuagint, pharmakeia φαρμακεία, which are the agents (drugs, either natural or lab-produced) with unique properties, that give the users awareness in the spirit world. This was introduced on a massive scale in the 1960s and 70s in the U.S. and U.K. countercultures, to the point that it could be called recreational sorcery. I've written a lot about this here so I won't elaborate much, suffice it to say that we in those days had been seduced (through a Trojan Horse from Hell) into the demonic realm (though it initially purported to be the divine), and there was much demonic activity entering the human sphere resulting from that.
This is something that shamans, occultists, spiritualists, pagan worshippers, witch doctors, etc are well aware of, as well as some worldly users – getting high on these particular agents, the psychedelics or entheogens is having your awareness elevated into that realm. Not only was it a thing back then, but now in 2024, not only is extremely potent grass widely legal, but the other psychedelics – psilocybin mushrooms, LSD, DMT, ayahuasca, peyote, etc – and related compounds like MDMA and ketamine are either legal or touted as therapeutic (meaning to most, healthful)! I am referring only to the psychedelics, not traditional legitimate medicines.
What this means is that our zeitgeist – spirit of the age, or collective consciousness – is increasingly saturated with demonic content, influence, and presence. The increase of violence – its being considered legitimate and warranted politically, as well as our culture generally becoming much more so, vulgarity and sexual "openness", trans and LGBTQ+ and gender fluidity, governmental disinformation and increasing oppressive control, all are indicators of a massive incursion of demonic spirits into our society and lives. We don't recognize the "demonic" aspect of them as these things have become so normalized.
Those who are not shielded and armed by the seal of God and the indwelling of His Spirit and word are all vulnerable to this rising tide of madness.
Yes, in the pockets of mostly sane people – mostly Christian or very conservative enclaves – bizarre evil-driven behaviors can be discerned as demon-influenced, but our world is being groomed for a fuller infilling, the end of which is the silencing of the greatly offending presentation of the life-giving Gospel of Jesus Christ, and the targeting for extermination of the entire Christian community globally the planned finale of the evil one – in the midst of which atrocity our Saviour and King will return to rescue and raise us unto Himself and execute His wrath on those who hurt His precious bride.
Some thoughts on this in eschatological terms:
UNCOVERING PROPHETIC DETAILS IN REVELATION: Restoring confidence in the applicability of John’s Apocalypse, the final prophecy: