From Inner.org, the writings of Rabbi Ginsburgh (hassidic kabbalist in Israel), from his weekly mailing list (available to all, click here):
"In general there are 5 stages in the revelation of the wisdom of Kabbalah, each stage appearing (one might say, even encoded) within a particular text."
Four: The Tree of Life
By the work of the Ramak and others on the Zohar many of its mysteries were revealed. Nonetheless, the deepest teachings and concepts could not be fathomed by the human mind alone if it were not inspired from above by the Almighty. The Holy Ari, the acronym for Rabbi Isaac ben Shlomo Luria, was the new soul sent from above and chosen to reveal these. The Ari arrived in the Ramak’s hometown, the holy city of Safed , on the same day that the Ramak passed away and was being buried.
Beyond having the intellectual skills and knowledge of his predecessors, the Ari merited to receive a completely new revelation of wisdom from Heaven. Though this new wisdom could be taught independently, the Ari chose to “enclothe” it as an interpretation of the Zohar, that is, to teach it in context of the language and style of the Zohar. The Ari’s teachings were then compiled into the text titled The Tree of Life (Eitz Chayim). So all-inclusive and penetrating were the Ari’s teachings, that a student of Kabbalah who has not yet learnt the fifth level of Kabbalah, will tell you that the classic work for the study of Kabbalah in our generations is The Tree of Life.
BS"D - בסיעתא דשמיא