The Talmud relates the words of Rabbi Yochanan ben Zakkai uttered in the moment just prior to his passing: "I don't know in which direction I will be led," (i.e., whether he would go to Heaven or the opposite).

How can we understand this uncertainty from a spiritual giant such as Rabbi Yochanan? Because of his preoccupation with fulfilling his mission in this world - in which he was engaged up until his last second - Rabbi Yochanan had no time to examine or evaluate - let alone pride himself in - his spiritual accomplishments.

He knew that if he would squander even one moment that could have been devoted to performing mitzvos or strengthening his knowledge of Torah, he could be considered rebellious against G-d by not fulfilling his mission to the best of his ability.

Toras Menachem 5712, vol. 1, p. 332