"Masekhet Atzilut... is a Baraita of the wisdom of the Kabbalah of the ancient Sages from the days of King David (upon him be peace), discussing the subject of the Ten Sefirot. It was written by Y'ari'shiya, Zecharia, and Eliyah the sons of Yeruham, mentioned in 1 Chronicles 8."
The Zoharic Kabbalah did not invent Kabbalah; but it did radically reinterpret and replace the older schools of Kabbalah that had been in existence since Biblical times. Masekhet Atzilut is an excellent example of the "older" Kabbalah that predates the Zohar and Zoharic influence.
Masekhet Atzilut therefore discusses certain secrets of the prophetic and practical Torah which, for the most part, were either concealed or forgotten by the later generations of the philosophical Kabbalists.
These subjects include: