When a person believes that one has succeeded in achieving closeness to G-d and understanding of Him, it is a sign that the person does not know anything at all. Otherwise, the person would understand that one is very far from G-d and knows absolutely nothing, because G-d’s greatness is without limits.
Based on Rebbe Nachman, Likutei Moharan I: 63
The Essential Rabbi Nachman, by Avraham Greenbaum
We may speak of Hashem, but we know absolutely nothing. It is said that the goal of all knowledge [of Hashem] is to realize that one knows nothing. But even this cannot be attained… A person knows nothing, and still cannot perceive one’s ignorance. For there is always a degree of “not-knowing” on a level that lies beyond one’s perception… (Rebbe Nachman, Sichot HaRan 3)
There is something called takhlit hayedia, the greatest point in knowledge achievable by a person. This is the point where a person reaches “asher lo neida,” that one does not know. This is when a person realizes truthfully that one has no knowledge whatsoever. Only then has one reached the apex of knowledge.
Now, we are not speaking of people who truly know nothing. We are speaking about those who have gotten to such a high degree of knowledge in kedusha (holiness) that they go higher in heavenly knowledge. At this point, they understand that all this heavenly knowledge and all of creation are nothing compared to Hashem. At this point, they realize they are literally nothing. They have gotten to the upper point of zero.
Based on Rebbe Nachman’s Soul