Mysticism is simply regression to an infantile state of consciousness.
Let's start with Sigmund Freud. As you probably know, Freud started out as a medical doctor. He practiced medicine in Vienna. His specialty was the brain and nervous disorders.
As part of his postgraduate education, Freud studied in Paris with Jean-Martin Charcot, the leading neurologist of his day. Back in Vienna, Freud collaborated with Josef Breuer and eventually wrote a book with him. Both Charcot and Breuer experimented with medical hypnosis. But Freud concluded (Introductory Lectures on Psycho-Analysis):
But worse than its capricious nature was the lack of permanence in the results; after a time, if one heard from the patient again, the old malady had reappeared or had been replaced by another. Then one could begin to hypnotize again. In the background there was the warning of experienced men against robbing the patient of his independence by frequent repetitions of hypnosis, and against accustoming him to this treatment as though it were a narcotic.
Freud's only use for hypnosis was as a way to get patients to talk more freely. On the basis of listening to his patients talk freely, Freud formulated his theories about the way the human mind develops and works. He called this analysis of the mind psycho-analysis.
One theory concerned human psychosexual development, in which he observed that later developments build on earlier ones. But, he says -- and you can read this in his 1915 introductory lectures at the University of Vienna, which were published in English in 1920  -- there is a "danger of this development by stages." It's possible under some circumstances to undo later development and fall back. "Even those components which have achieved a degree of progress may readily turn backward to these earlier stages. Having attained to this later and more highly developed form, the impulse is forced to a regression when it encounters great external difficulties in the exercise of its function, and accordingly cannot reach the goal which will satisfy its strivings."
So initially he used the word regression to mean an involuntary return to an earlier stage of development, brought about by frustration.
In the 1920s he turned his attention to the culture as a whole rather than the psyche of the individual in particular. One book he published during his period, in 1927, was The Future of an Illusion. Now keep in mind that Freud defines himself as a "godless Jew."  So this is a "godless Jew's" take on religion. And it's not a very good book. But Freud must have liked it, because he sent a copy to a friend of his, the French writer Romain Rolland.
Rolland wrote back with his view of religion, which was quite different from Freud's. Rolland described a sense of timelessness and limitlessness, which he called an "oceanic feeling," and which he said for him was always present -- in the background of experience, it seems. And Rolland believed this oceanic feeling was in fact the real source of all religions.
Freud responded to Rolland's view in chapter 1 of Das Unbehagen in der Kultur, first published in German in 1929 and in English in 1930 as Civilization and Its Discontents.
Thinking about the oceanic feeling led Freud to conclude that "the adult's ego-feeling cannot have been the same from the beginning. It must have gone through a process of development, which cannot, of course, be demonstrated but which admits of being constructed with a fair degree of probability."  So he's saying that the sense of self, the sense of I-ness, that most people take for granted, hasn't been there all along. The "oceanic feeling" was the way all of us experienced reality before the formation of our egos.
And then Freud -- or more accurately, a second friend of his -- raises the possibility of deliberate regression effected by spiritual practice. This is in contrast to involuntary regression in response to frustration.
This is the way Freud describes deliberate regression: "Another friend of mine, whose insatiable craving for knowledge has led him to make the most unusual experiments and has ended by giving him encyclopedic knowledge, has assured me that through the practices of Yoga, by withdrawing from the world, by fixing the attention on bodily functions and by peculiar methods of breathing, one can in fact evoke new sensations and coenesthesias [a coenesthesia is just a bodily awareness] in oneself, which he [meaning the second friend] regards as regressions to primordial states of mind which have long ago been overlaid. He sees in them a physiological basis, as it were, of much of the wisdom of mysticism." 
So there you have it. Even in the 1920s, a few people were already aware that mysticism was essentially regressive. Freud was very careful with the way he worded this chapter, and he makes it clear that this mystical regression is not something he has personal knowledge of, but which emerges as a possibility from the experiences of his correspondents.
So what does this regressive state of consciousness look like?
We can start with Romain Rolland's description: "le fait simple et direct de la sensation de l'éternel (qui peut très bien n'être pas éternel, mais simplement sans bornes perceptibles, et comme océanique)," "the simple and direct fact of the feeling of the eternal (which may very well not be eternal, but simply without perceivable boundary markers, as if oceanic)." So a feeling of timelessness and limitless space.
Then we can add from Piaget the notion that the regressive state must exist before object permanence. Jean Piaget was the Swiss psychologist who experimentally investigated children's cognitive development. One early milestone he came up with was object permanence. This is the developmental stage where an object still exists in the mind of the infant even if it's not immediately available to the senses. Object permanence develops over the first few months of life outside the womb. Before the development of object permanence, it's likely that the infant's consciousness contains no notion of solidity and separateness. Everything is fluid and not separate from anything else.
And then, in terms of affective development, the infant is believed to exist in a narcissistic state, in which the libido is in love with the libido.
Putting these all together gives us a picture of a self-contained and undifferentiated experience. And this is exactly the same experience as is recognizable from the writings of the mystics: undifferentiated awareness enjoying undifferentiated awareness.
For example, in the Yoga Sutras of Patañjali, we read tadā draṣṭuḥ svarūpe 'vasthānam, "then the see-er abides in his own nature." (That's Yoga Sutra 1.3.)
And in the mystical traditions, reaching this regressive state is given various glamorous-sounding names, such as awakening, enlightenment, realization, etc., all of which refer to essentially the same thing: reconnecting with early, regressive states of consciousness.
So how can regressive states be entered? From the descriptions of human cognitive development and the possibility of the undoing of this cognitive development, the ways to reach it are almost obvious.
First you must abandon verbal, conceptual, or rational thought. And you can see if you study the spiritual literature that a lot of what are called "spiritual teachers" actively encourage the abandonment of verbal thought.
That's actually a two-edged sword. It does take you closer to regression. But it also leaves you vulnerable to deception. If you abandon critical thinking, you have no way of telling whether what you're being told is deceptive or not. We'll come back to these dangers in a moment.
For now, let's carry on. Continuing with the journey toward regression, having reached a preverbal layer of the mind, you must then continue in that direction until there's absolutely no movement left in the mind at all.
A well-documented example is Bernadette Roberts. I used to admire her at one point, but now I'm not so sure. Her practice was simply enjoying inner silence, sometimes for hours and hours at a time, every day when she could manage it, and continuing this practice for decades. She writes about her enjoyment of silence at the start of her first book: 
"Through past experience I had become familiar with many different types and levels of silence. There is a silence within, a silence that descends from without; a silence that stills existence and a silence that engulfs the entire universe. There is a silence of the self and its faculties of will, thought, memory, and emotions. There is a silence in which there is nothing, a silence in which there is something; and finally, there is the silence of no-self and the silence of God. If there was any path on which I could chart my contemplative experiences, it would be this ever-expanding and deepening path of silence."
And, of course, if you're familiar with Eastern traditions of meditation, you know that they have many practices for cultivating still, silent, somewhat trance-like states of consciousness. These also lead toward regression.
At this point, I'd like to introduce the biological notion of hormesis. Hormesis is a phenomenon in which something is beneficial in small quantities but harmful in large quantities. Examples would be stress or physical exercise. Small quantities can actually stimulate you; large quantities will exhaust you.
Regression gives you a motionless vantage point on adult consciousness. This vantage point brings about clearer awareness of the way your mind works.
That awareness can be therapeutic. It often seems to me that marriage counselors would be out of work if both parties cultivated self-awareness!
And stillness is good in our frenetic, over-stimulated society.
The problem comes when you make stillness, awareness, and abiding in regressive states into goals in themselves, and you begin to think that this is the purpose of life.
If stillness and awareness become goals, my observation is that you become less and less functional at the ordinary business of living.
Back to Bernadette Roberts again, just as one example among many. She describes being unable to hold down a job as a schoolteacher: 
"In a school where I taught it was a standing joke that whenever I lectured, anybody and everybody could crawl in and out the windows without my noticing a thing -- as if I were blind or something! I regarded these antics as attention-getting behaviors and decided if I ignored them they would eventually wear themselves out -- which proved to be true. But if I had the patience to win out, the administration did not. In the long run, I was fired for allowing this monkey business, and in leaving could not refrain from suggesting that instead of a teacher they hire a zookeeper."
So she'd been trained as a schoolteacher but couldn't function as one. Then at home, caring for her own children, she absent-mindedly left her slippers in the refrigerator.  Her fixation on inner progress had left her less and less able to function in the real world.
And you can read other accounts, if you search around for biographies, of other so-called "spiritual teachers" being unable to carry out the duties they were trained for. Sometimes they excuse this lack of functionality as though it were some kind of advanced "spiritual progress."
The description of the regressive state as awareness in love with awareness sounds good, but it's a dead-end. Retreat into yourself isn't growth.
The error compounds itself if you take your obsession with regression and awareness of the pre-personal layers of consciousness and turn it into a value system. You devalue the personal and the real and the ordinary. Doing that is to impose a neurotic and toxic value system on top of your experiences.
Retreating from the world into one's own interior is sometimes thought of as typical of Eastern mysticism, but it does exist in Christianity, too. I'm thinking now of the seventeenth-century movement that came to be known as Quietism. (This was not their name for themselves; it was a name imposed by outsiders.)
The leader of the Quietists was a priest named Miguel de Molinos. Among the statements attributed to de Molinos were propositions such as, "It is necessary that man reduce his own powers to nothingness," "To wish to operate actively is to offend God," and "Vows about doing something are impediments to perfection."  You can see that these lead to, and eulogize, the infantile state. In this scheme of values, you're supposed to do nothing -- which is completely infantile and regressive. You've abandoned your adult powers and responsibilities.
And then to take an example from Eastern mysticism, an American social worker joined an Eastern regression cult and later explained: "Before I left for Pune, I was a very capable, articulate, professional woman. When I got back I was in a totally hypnotic state. I was absolutely helpless." 
We'll come to this hypnotic business in a moment.
I've said that becoming fixated on regressive states will make you less functional in your ordinary life. But abiding in regressive states actually makes you more of a danger to those around you than to yourself.
If you make regression your permanent home, you operate without a super-ego and without the normal societal restraints. Encouraging other people to abandon critical thinking leaves them vulnerable. The combination of vulnerable followers and a leader without scruples almost inevitably leads to exploitation and abuse.
The social worker just mentioned exemplifies this dark side of regression: a tendency for exploitative cults to form around the leader.
Such leaders are sometimes described as "malignant narcissists." They are totally in love with their own consciousness, and they have none of the normal social limits.
They could also be described as "traumatized infants with hypnotic powers." Traumatized infants because that's what's left without a super-ego. And hypnotic powers for the reasons I'm about to explain.
An individual with access to regressive states automatically draws others to themselves. They have none of the usual adult concerns, so they appear very free. And people are drawn to this sense of freedom from reality by a kind of "pull." This is already a sort of trance induction.
But then a surprising number of cult leaders have also deliberately studied hypnotic techniques. These techniques are little known among the general public. They don't involve the stereotypical verbal suggestions. They're done silently with eye gazing.
If you look on YouTube, there's an Italian hypnotist named Marco Paret who teaches the eye-gazing induction today. 
The technique starts with learning to stare without blinking for longer and longer periods of time -- up to several minutes. This practice actually has a name in India. It's called trāṭaka. From the very fact it has a name, we can conclude that eye-gazing induction has been known in India for much longer than in the West.
In some old books, it's called "fascination" rather than "hypnosis." And it gives the practitioner psychological power over other people. This fascinating gaze forms a basis for mind-control. Most people are completely unaware that there even is such a thing as fascination, and their lack of knowledge is part of what makes cult leaders so dangerous.
(Also on YouTube, if you're interested, you can view the testimony of "Sally Jane," who is a former professional hypnotist and who is now a Christian.  )
The fact that people addicted to regressive states can't earn a living in the normal world makes it all the more tempting for them to gather supposed "students" around them, in a relationship that benefits the supposed "teacher" much more than the "students."
So far I've given an explanation of regression cults that is entirely natural. But there is also a spiritual side to all this. I'll start with two quotations from the apostle Paul.
In Second Corinthians 10:3 we read: "For though we walk in the flesh, we do not war after the flesh." So the natural side exists, and we move in that world, but there's a spiritual side to this battle that's actually more important than the natural side.
And again in Ephesians 6:12, "For we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places." So there it is again: wrestling happens in the natural world, but it is the spiritual side that really matters.
When we discuss spiritual warfare, Neil T. Anderson points to the tendency of people to adopt extreme positions. He says, "The tendency is to polarize into a deliverance ministry that ignores the other realities of life, or to a psychotherapeutic ministry that ignores the reality of the spiritual world." 
We can see from Scripture that they're both there. "We walk in the flesh," says Paul, but "we do not war after the flesh." The natural is there, but it's the spiritual battle we need to be aware of.
And this battle is taking place inside our own minds. Neil T. Anderson again: "The primary battle is between the kingdom of darkness and the kingdom of God, between the Antichrist and the Christ, between the father of lies and the Spirit of truth; and we are in that battle whether we like it or not. The primary location of that battle is our minds. Either we believe the lies that keep us in bondage or we believe the truth that sets us free. So we define spiritual warfare as the battle for the mind." 
So there's the pointer to the essence of it. Regression cults maintain their grip on their followers by telling them lies. On a spiritual level, regression cults are about deception.
Neil T. Anderson observes: "Satan's strategy is always deception."  In John 8:44, Satan is called "a liar and the father of lies." And the essence of Satan's strategy is also pointed to in Revelation 12:9. "The great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world." There's the deception again. Satan's strategy is deception.
The solution, says Paul, is to "put on the whole armour of God" (described in detail in Ephesians 6:11-17). This includes the truth, the Word of God, and also the name of Jesus. We read in Luke that when the seventy returned, they said, "Lord, even the devils [which is the same word in Greek as 'demons'] are subject unto us through thy name" (Luke 10:17). And "thy name" is the name of Jesus.
 Sigmund Freud, A General Introduction to Psychoanalysis, trans. G. Stanley Hall (New York: Horace Liveright, 1920), p. 295.
 Sigmund Freud, letter to Oskar Pfister, October 9, 1918.
 Sigmund Freud, Civilization and Its Discontents, trans. Joan Riviere (London: Hogarth Press, 1930), p. 11.
 Ibid., p. 22.
 Bernadette Roberts, The Experience of No-Self: A Contemplative Journey, Revised Edition (Albany, N.Y.: State University of New York Press: 1993), p. 19.
 Bernadette Roberts, The Path to No-Self (Albany, N.Y.: State University of New York Press: 1991), p. 140.
 Bernadette Roberts, The Experience of No-Self.
 Coelestis Pastor, 1687, https://www.papalencyclicals.net/innoc11/i11coel.htm
 Win McCormack, "Bhagwan’s Mind Control," New Republic, April 12, 2018, https://newrepublic.com/article/147902/bhagwans-mind-control
 https://www.youtube.com/user/neurolinguistic. Marco Paret credits Erminio di Pisa as his teacher, though this author is little known outside Paret's references to him. Marco Paret also narrates that his friend Max Tira knew of a barber, Virgilio Torrizzano, who had mastered the art of the hypnotic gaze induction (which they often call "fascination" or "magnetic fascination" in contradistinction to "hypnosis"). He goes on to say that Torrizzano had learned fascination from a master, who may have been in the lineage of Alfred Édouard d'Hont, stage name Donato. Marco Paret believes that the lineage ultimately descends from Franz Anton Mesmer himself. The oldest published source they refer to is La fascination magnétique by Édouard Cavailhon (Paris: E. Dentu, 1882). This places Donato as the central figure of the narrative and includes a lengthy preface by him. Marco Paret also refers to the works of James Coates, William Walker Atkinson, and Jean Filiatre.
 Heal and Restore, "Ex Hypnotist to Jesus Christ. The dangers of hypnosis," https://www.youtube.com/watch?v=xxm3Ot3Agm0
 Neil T. Anderson, "Finding Freedom in Christ," in C. Peter Wagner, ed., Supernatural Forces in Spiritual Warfare: Wrestling with Dark Angels (Shippensburg, Pa.: Destiny Image, 2012), p. 124.
 Neil T. Anderson and Timothy M. Warner, The Essential Guide to Spiritual Warfare (Bloomington, Minn.: Bethany House, 2000), p. 34.
 Ibid., p. 128.