Baal HaSulam, “A Speech for the Completion of The Zohar”: And since the degree of the authors of The Zohar is at the full level of the 125 degrees, they cannot be attained prior to the days of the Messiah. It follows that there will be no common attainment with the authors of The Zohar in the generations preceding the days of the Messiah. Hence, The Zohar could not be revealed in the generations before the generation of the Messiah.
That explains why Kabbalists hid The Book of Zohar. After all, there was no need in it for all the generations before ours. To give them this book would be like giving school children or even students the materials at the level of Nobel laureates. This would only confuse people. For example, if we tried to teach children the theory of relativity or other complex concepts, they would not be able to digest it and perhaps it could even damage their minds.
A person gradually acquires the essential means to approach difficult materials. It’s not easy. Some frontiers of science are available only to a few people in the world to understand, and not all of them are also able to work with the material, meaning to develop it further. Not everyone’s mind can withstand it.
Therefore, people should not be given an unripe fruit, or rather the fruit for which they are not yet ripe. Everything needs to be learned gradually. It’s said: “Don’t put a stumbling block before a blind person” because he will fall. Give him a chance to develop in stages.
The authors of The Zohar knew what they were doing. They were on the spiritual level inherited from the Temple, on the degree of love for the other as for oneself, and due to love, they attained the Creator concealed in it. They describe their attainment in The Book of Zohar, explain how they are organized between themselves, and they reveal the Creator in accordance with it.
However, others still could not work in this way because then they were entering a period of two thousand years of exile. That is why the Kabbalists hid The Zohar until the present day. But nowadays, this book is revealed to everyone.
From the 4th part of the Daily Kabbalah Lesson 12/23/13, Writings of Baal HaSulam