Kabalah . Much of the formal structure of what we have from Isaac Luria comes from the pre-Socratics, Plotinus and Mani (founder of Manichaeism--the faith that Augustine broke away from).  It does not seem all that insightful when you know from where it comes.  On the other hand  once you have the formal structure, it seems often the mystics themselves had  some great insights. It is not a settled question me.

I made a detailed study of this once. Mainly the idea of the ten sepherot comes from a disciple of Plato. The contraction {"tzimtzum"} was from the presocratics. The "sparks of holiness" from Mani. Adam Kadmon also from Mani.

And most of what passes for divine spirit by so called kabalists seems to me to be mainly kelipat Noga. That is the Middle Zone [heichalai hatmurot] between holiness and unholiness that gives one great powers and knowledge about peoples secrets.

But when you have people that were  ascetics like Bava Sali then it is fairly simple to see that their insights came from their sincere service to God and not from learning Kabalah. In fact it seems to me that what every kabalah they learned had little to do with their ability to give blessings.

Mainly I think that Kabalah is a way of conceiving spiritual reality. And when one gets into it then the reality becomes real. It is like Kant's idea of the representation   of "the thing in itself." The representation is half supplied by the object and half by the observer. That is the believing in it makes it real. Not just the Kabalah but any spiritual reality system has this quality of being able to absorb people that believe it into itself.

Appendix: (1) Plato's disciple did not actually invent ten sepherot. At first there were nine. Only in the Middle Ages was a tenth added to account for the precision of the north star.  And you can see this scheme in the end of the Eitz Chaim. And while we do not think that the Ari was learning Manichaeism but all of these ideas were common in the Middle Ages when people had been learning Plotinius and Mani's beliefs were also wide spread and almost became the primary world religion at one time. All these ideas were put into the Zohar and that is where the Ari found them.

(2) I do not mean to deny the validity of the Ari. Rather I simply say he was seeing the Torah through the worldview of the time of the Zohar. But in any case if one want to learn Torah I think the best option is simply the traditional Oral and Written Law. That is the Old Testament and the two Talmuds. Not Kabalah.

(3) One of the central beliefs of Manichaeism was the notion that every human being had two warring souls: one that was part of the Light, and another that was evil. This was itself based on Zoroastrianism.  

(4) According to Mani through lust and the sin, the Darkness tries to imprison more and more bits of Light within matter. 

(5) Seeing how much of Kabalah incorporates beliefs of ancient religions made it less interesting to me. Unless I would have thought that Mani was a true prophet. I could keep on making excuses but at some point it seemed more interesting just to go back to learning straight authentic Torah and leave the deluded with their delusions.

(6) To get  better idea of what Torah is about I think it makes more sense to look at Maimonides and Saadia Gaon, Ibn Gavirol, and the Duties of the Heart. Though Ari still gives very important insights, still I would not take that as standard.

(7) Another aspect of Manichaeism that became an important part of  the teachings of the Ari is the שם ב'ן in which there was the breaking of the vessels and then the rebirth of the name מ''ה החדש Adam Kadmon after the tikun of the vessels in the form of the the sepherot that is well known.

In Mani we also find the three stages-the first creation. The breaking of the vessels. Then the second creation with Adam Kadmon being reborn. Then the final Redemption. All very well defined in Kabalah and in Mani.

For me this makes the approach of Saadia Gaon and the basically rationalist Jewsih philosophers of the Middle Ages more interesting than Kabalah. Though I have the greatest respect for the Ari and genuine Mystics still their visions do not define the worldview Torah for me.

(8) I spent a great deal of time learning the Ari and I think  that after a good solid background  in Talmud the Ari can serve as a kind of conduit for a kind of Divine light. So I do not want to discount its importance. But by and large it just leads people to delusions. That is its effect on 99% of those involved with it.  It gains mastery over men's minds by the astonishing completeness, minuteness, and consistency of its assertions. They lose themselves in it.

(9) For me I should say I found learning the books of the Ari and the Gra in Kabalah to be very helpful. The trouble without these books the world is drained of its mystery and magic. It becomes a secular  world. The world of  The Guide for the Perplexed is a secular world. The world of the Ari is full of holiness and mystery. And  I learned to find the magic and holiness in everything --especially Physics which to me reveals the greatness and wisdom of God.