I was involved in mysticism, both Christian and Buddhist, for many years.
Then I came across Karl Barth's statement that Christian mysticism conveys "a different spirit than the Spirit of Christ." (I wish I could find a source for that quotation, but I can't.)
I'd noticed the same thing as Barth. The whole flavor of Christian mysticism is different from the flavor of the New Testament.
Here are the four problems I see with mysticism.
Mysticism makes promises about the end results that simply aren't true. You're told that you'll realize oneness, you'll realize the nature of ultimate reality, you'll realize nonduality (that's a popular one today), you'll attain union with God, and so on.
However you word it, these promises are false. You're not really attaining union with God or realizing nonduality. All you're doing is regressing into early layers inside your own mind.
A few people already knew this a hundred years ago. You can read how it works in Chapter 1 of Freud's essay Civilization and Its Discontents. (In German, the original title was Das Unbehagen in der Kultur.)
Freud's friend Romain Rolland routinely experienced a sense of timelessness and limitlessness he called the "oceanic feeling." Now, all that feeling really is, is very early consciousness. It consists of a sort of undifferentiated narcissistic bliss. And all subsequent development -- language, the intellect, and so on -- takes place on top of the most primitive layer.
But it's possible to get down to the original oceanic feeling. Freud compares this with the way the city of Rome was built up in layers. Rome started out as just a small square settlement, which we call Roma Quadrata. And even today, after centuries of development, you can still see traces of the original Roma Quadrata here and there. Freud compares this with human psychological development. It's still possible to regress back to our original, oceanic feeling. And another friend of Freud's then confirmed that that's what yogic exercises aim to do -- to regress to the earliest layers of the mind.
So this idea that you're attaining some sort of spiritual union is a lie. You aren't. You've been deceived.
I'm not saying that spiritual practices are useless. My point is more subtle than that. There are useful elements in spiritual disciplines. Periods of stillness and solitude can be refreshing. Routinely sitting still with your eyes closed alleviates anxiety -- which is fine, provided you view it as a kind of daily hygiene, the psychological equivalent of brushing your teeth. Even regression can be therapeutic. And awareness is valuable. In fact, you must have some kind of self-awareness before you can repent.
My point is that it's a question of emphasis and values. And it's a question of honesty. Spiritual practices can be helpful if they're understood properly and kept in proportion. It's deceptive language about the outcome that I'm pointing to as a problem.
The reason that Christian mysticism seems so foreign to the New Testament is that is is foreign to the New Testament.
How that happened is that, in the third century, a long time after the New Testament, a philosopher named Plotinus created a system of philosophy we call Neoplatonism. (That's wasn't his name for it; that's our name for it.) In Neoplatonism, there's no Creator and creation. Instead, Plotinus starts from what he calls the One. And this One produces the apparent multiplicity of visible things by a process he calls emanation. And the highest purpose of life is to contemplate the One until you find your way back to the One.
Unfortunately some Christians were unwittingly influenced by Neoplatonism. They adopted some of Plotinus' ideas, but dressed them up in Christian vocabulary. Instead of union with the One, they started talking about union with God. According to them, the idea was to withdraw your interest from visible reality, contemplate "God" alone, and so find your way back to "union with God." That's their way of putting of it.
Historically, you can trace a line from Neoplatonism through Gregory of Nyssa to the writer who calls himself Dionysius the Areopagite. And from there, these ideas create the entire Christian mystical tradition, including the Cloud of Unknowing, John of the Cross, and Meister Eckhart. For Meister Eckhart, finding your way back to the One is just expressed as finding your way back to the ground of your being. And all along, this mystical tradition is just Neoplatonism in disguise.
Part of that falsehood is the highly questionable habit of equating God with your own early consciousness. That's certainly not what the Old Testament means by God.
So what we see is that the whole Christian mystical tradition is suspect. It looks like Christianity, but it isn't. It's a foreign philosophy pretending to be Christianity.
Mysticism may not be so foreign to Eastern religions. But even here, it advocates what is essentially emotional disengagement from life. And that's not a healthy response to life's problems. The prayer of recollection resembles the non-Christian practice of pratyāhāra (Yoga Sutras 2.54), the withdrawal of the senses from the external world.
My observation is that people who think life's goal is to reach regressive inward states tend to become less and less functional in their external lives.
I won't mention any names, but I'm thinking of one well-known Christian mystic of the twentieth century, a woman whose books I used to admire. If you read about her life, you'll learn that her career failed, her marriage failed, and her finances failed.
If you completely withdraw your attention and interest from your surroundings, that's what happens. Your ability to function is impaired.
This is a bit controversial, but when awareness goes deep in to the mind, people sometimes develop strange, occult powers. And the temptation they almost inevitably fall into is using occultism for selfish purposes -- personal gratification.
What makes this temptation all the worse is that people who've bought into the deception tend to develop a sense of specialness. They think they're beyond normal rules for human behavior. Romans 3:23 says, "All have sinned and fall short of the glory of God." And that applies to mystics and occultists as much as anyone -- even if they think it doesn't.
Neoplatonism easily morphs into occultism. Jesse Finley Hurley's Sorcery (introduction, pdf) claims that magic can be explained scientifically by this philosophy. In Hurley's version, orders from the "conscious mind" are delivered via the "unconsciousness mind" out to the common universal mind. Hurley cites phenomena such as telepathy and hypnosis in support of his theory. The Bible condemns all such occultism: "There shall not be found among you any one that maketh his son or his daughter to pass through the fire, or that useth divination, or an observer of times, or an enchanter, or a witch" (Deuteronomy 18:10). Sorcery (Greek φαρμακεία) is listed as a sin in Galatians 5:20.
So the fourth part of the case against mysticism is this tendency for it to degenerate into occultism.