Baal HaSulam, “A Speech for the Completion of The Zohar“: Throughout the generations those who ascended and were rewarded with Dvekut were few, but in the generation of the Messiah, everyone can reach Dvekut and attainment.
The restrictive conditions throughout history in every generation, in every state and circumstances where Kabbalists lived, didn’t allow them to reach the end of correction, and so their attainment and revelations were not complete.
The one exception in history is the case of Rabbi Shimon and his students. This small group was especially allowed to reveal all of reality, the process, and the goal so that they would pass these basics from the beginning of the exile, from the state before the destruction of the second Temple, from the corrected Israel to the nation of Israel that is totally corrupt and dispersed. Thus we will be able to get back on the path that will lead us to the good state.
So in every generation, there were only certain individuals who attained the spiritual world, while in our generation anyone can. There are no limitations from above and everything depends on a person’s ability to connect to the friends. The individual is powerless and incapable of anything, but he can connect with the friends in the group and thus reach the goal.
From the 4th part of the Daily Kabbalah Lesson 12/19/13, Writings of Baal HaSulam