Eliphas Levi about Kabbalah in the book "Transcendental Magic: Its Doctrine and Ritual":
"On penetrating into the sanctuary of the Kabbalah one is seized with admiration at the sight of a doctrine so logical, so simple, and, at the same time, so absolute. The essential union of ideas and signs; the consecration of the most fundamental realities by primitive characters; the trinity of words, letters, and numbers; a philosophy simple as the alphabet, profound and infinite as the Word; theorems more complete and luminous than those of Pythagoras; a theology which may be summed up on the fingers; an infinite which can be held in the hollow of an infant’s hand; ten figures and twenty-two letters, a triangle, a square, and a circle; these are the entire elements of the Kabbalah.
These are the component principles of the written Word, reflection of that spoken Word which created the world! All truly dogmatic religions have issued from the Kabbalah and return therein; whatsoever is grand or scientific in the religious dreams of all the illuminated, Jacob Boehme, Swedenborg, Saint Martin, &c., is borrowed from the Kabbalah; all masonic associations owe to it their secrets and their symbols.
The Kabbalah alone consecrates the alliance of universal reason and the divine Word; it establishes, by the counterpoise of two forces apparently opposed, the eternal balance of being; it only reconciles reason with faith, power with liberty, science with mystery; it has the keys of the present, past and future!
To become initiated into the Kabbalah, it is insufficient to read and to meditate upon the writings of Reuchlin, Galatinus, Kircher, or Picus de Mirandola; it is necessary to study and to understand the Hebrew writers in the collection of Pistorius, the Sepher Yetzirah above all; it is necessary also to master the great book Zohar, read attentively in the collection of 1684, entitled Kabbala Denudata, the treatise of Kabbalistic Pneumatics, and that of the Revolution of Souls."

Manly Palmer Hall
"The Transcendentalism of the Qabbalah is Founded Upon the Ancient and Magical Formula of King Solomon. Who has Long been Considered by the Jews as Prince of Ceremonial Magicians." [Manly Palmer Hall (1901-1990)]

The Tree of Life, of course, is mentioned many times in the Bible, from the beginning of the Book of Genesis to last part of the Book of Revelation. One who eats from this Tree, according to the Bible, is given "eternal life." The above diagram of the Tree of Life is part of an ancient tradition called "Kabala." Other spellings of the word include, kabbala, cabala, cabbala, quabala, and qabbalah. This is generally known as a special Hebrew tradition, the occult philosophy of certain Jewish rabbis, especially in the Middle Ages, based on a mystical interpretation of the Scriptures. Many scholars believe that the tradition is much older; that it traces back to the ancient Egyptians and Sumerians.

Connections to Ancient Sumeria
Over one hundred years ago, clay tablets were found in present day Iraq. They were artifacts with Cuneiform writings and pictographic carvings, left by the ancient people of that region. The Assyrian and Babylonian empires were located in that area long ago, but an even more ancient one preceded them, called Sumer, which in the Bible is called "Land of Shine'ar." According to scholar, Zecharia Sitchin, in Divine Encounters (page 7) -

Sumer (the biblical Shine'ar) was the land where the first known and fully documented civilization sprang up after the Deluge, appearing suddenly and all at once some six thousand years ago.

Some of the pictographic carvings have depictions similar to the Kabalistic Tree of Life. The oldest story in history, The Epic of Gilgamesh, was also found. Gilgamesh was a king, who was part god. He traveled to a far place, looking for the plant or tree that gave eternal life.

The Meaning of the Tree of Life
The Kabalistic Tree is sometimes called The Sephirotic Tree of Life. The circles or spheres are called sephiroth. A single circle or sphere is called a sephira. The teachings about the Kabalistic Tree of Life are very deep and complex. The ten spheres and 22 paths of The Tree generally represent the nature of the forces behind Creation on all levels, from microscopic to macrocosmic. The Tarot Cards are also associated with The Tree.

The Hebrew word ???? (Kabbalah) has a Gematria (numerical value) of 137. In Modern Hebrew, the root of Kabbalah ?-?-? can mean either "receiving" or "parallel". Kabbalah is generally taken to mean "the received tradition", which conveys the continuity of a tradition that has been passed down from generation to generation. Nevertheless, the earlier nuance of meaning is seen in the first appearances of its root in the Torah (Exodus 26:5 and 36:12), where it means "parallel" or "corresponding" rather than "receiving". It is used to describe the "corresponding loops", which, when clasped together, enjoined the two sections of the Tabernacle's ceiling. These loops were suspended directly over the veil that divided the Holy Place and the Holy of Holies. Symbolically, this is the threshold between the physical dimension and the utterly spiritual dimension. In other words, at the boundary line of the physical world, the number 137 emerges. The wisdom of Kabbalah is to find correspondences between the mundane and spiritual levels of reality. https://en.wikipedia.org/wiki/137_(number)

"No secrets are taught in Kabbalah. The wisdom of Kabbalah is called “the wisdom of the hidden,” not because it is secret in and of itself, but because it reveals things that were hidden before we began to study. It reveals everything that surrounds us.
However, the wisdom of Kabbalah is comprised of two parts: “flavors of the Torah” and “secrets of the Torah.” The flavors of the Torah investigate the structure of the spiritual worlds, man’s soul, and how one should correct oneself. Everyone is permitted to study that part. This material is written about in books of Kabbalah sold all over the world and translated into English, Russian and other languages. Anyone can learn the flavors of the Torah.
The “secrets of the Torah” is the hidden part of the Torah. Nothing is written about it in any book. That part is taught only after a person has acquired the flavors of the Torah, attained the structure of the spiritual worlds as well as one’s own completely, and recognized and partaken of the process of creation.
A person who has attained that level, where physical life and death do not exist, sees the entire process from beginning to end and is above our world. Then the secrets open up like innermost fountains, and we understand the laws that are at the basis of that system. Before that, we will not understand the meaning of those secrets, even if we heard or saw them." [Rav Michael Laitman]

"In the most basic level, Keter is seen as the concept of Cause, while Malkhut is the archetype of Effect, Since a cause cannot exist without an effect, and an effect cannot exist without a cause, the two are interdependent on each other.
The Sefer Yeizirah likens this to a "flame bound to a burning coal," A flame cannot exist without the coal. and the burning coal cannot exist without the flame. Although the coal is the cause of the flame, the flame is also the cause of the burning coal Without the flame, it would not be a burning coal Since Cause cannot exist without Effect.
Effect is also the cause of Cause, In this sense. Effect is the cause, and Cause is the effect.
Since beginning and end are inseparable, "their end is imbedded in their beginning, and their beginning in their end." [Aryeh Kaplan "Sefer Yetzirah The Book Of Creation"]

Ouspensky
"The study of the Name of God in its manifestations constitutes the basis of the Cabala. "Jehovah" is spelt in Hebrew with four letters, Yod, He, Vau, and He. These four letters have been given a symbolical meaning. The first letter expresses the active principle, initiative; the second, the passive principle, inertia; the third, equilibrium, "form"; and the fourth, result or latent energy.
The Cabalists affirm that every phenomenon and every object consists of these four principles, i.e., that every object and every phenomenon consists of the Divine Name. The study of this name (in Greek the Tetragrammaton or the word of four letters) and the finding of it in everything constitute the chief aim of Cabalistic philosophy.
What is the real meaning of this? According to the Cabalists, the four principles permeate and compose each and every thing. Therefore, by finding these four principles in things and phenomena of quite different categories, between which he had previously seen nothing in common, a man begins to see the analogy between these things. And gradually he becomes convinced that everything in the world is constructed according to the same laws, according to the same plan.
From a certain point of view, the enriching of the intellect and its growth consist in the widening of its capacity for finding analogies. The study of the law of the four letters, or of the Name of Jehovah, can therefore constitute a means for widening consciousness. The idea is quite clear. If the Name of God is really in everything (if God is present in everything), then everything should be analogous to everything else, the smallest part should be analogous to the whole, the speck of dust analogous to the Universe, and all analogous to God. "As above, so below."
Speculative philosophy arrives at the conclusion that the world undoubtedly exists, but that our conception of the world is false. This means that the causes of our sensations which lie outside ourselves really exist, but that our conception of these causes is false. Or, to put it in another way, it means that the world in itself, i.e., the world by itself, without our perception of it, exists, but we do not know it and can never reach it, because all that is accessible to our study, i.e., the whole world of phenomena or manifestations, is only our percept of the world.
We are surrounded by the wall of our own percepts and are unable to look over this wall into the real world. The Cabala aims at studying the world as it is, the world in itself. The other "mystical" sciences have precisely the same object.
In Alchemy, the four principles of which the world consists are called the four elements. These are fire, water, air, and earth, which exactly correspond in their meaning to the four letters of the name of Jehovah.
In Magic, the four elements correspond to the four classes of spirits—spirits of fire, water, air, and earth (elves, water-sprites, sylphs, and gnomes).
In Astrology, the four elements correspond, very remotely, to the four cardinal points, the east, the south, the west, and the north, which, in their turn, sometimes serve to designate various divisions of the human being.
In the Apocalypse, they are the four beasts, one with the head of a bull, the second with the head of a lion, the third with the head of an eagle, and the fourth with the head of a man.
And all these together are the Sphinx, the image of the four principles merged into one." [A New Model of the Universe by P D. Ouspensky]

See Also

Bible
Ein Sof
Hinduism
John Ralston Skinner
Kabbala - Introduction
Kabbala - The True Science of Light
Kabbalistic Tree of Life
Sephiroth
seven Spirits
Tree of Knowledge
Vedas