Why was Moses denied entrance into the Holy Land (for failing to listen to G-d’s instructions at Meribah-Kadesh when wandering in the desert), when Moses obeyed so many of G-d’s commands and did so much for the people of Israel.

In addition to this specific question, what do the authorities and commentaries say about the concept of punishment? Can one never redeem oneself after committing certain offenses, no matter what other (good) deeds one does?

Although the topic you raise is very complex, I will try to explain the basic foundations of punishment and repentance. Without punishment for our actions, life would be miserable! You see, being punished (or judged) is a sign that our every action is observed by G-d, and is meaningful. By the way, it goes without saying that without punishment for the evil that we do, we cannot get rewarded for the good that we do. Either our actions matter or they don’t. As you can see, it is only because of the fact that G-d punishes and rewards us they life has any real meaning!

In regards to your question as to whether or not there are sins so heinous that one cannot achieve repentance for them, the answer is yes and no. Let me explain: The Jerusalem Talmud (first chapter of Peah) states quite unequivocally that there is nothing that stands in the way of repentance. On the other hand we find statements by the Sages concerning certain sins that cannot be repented for. The classic philosophical work Reishis Chachma (Gate of Holiness 17: 16-17) answers this seeming contradiction by stating that the type of repentance that the Jerusalem Talmud was referring to when it said that nothing stands in the way of repentance was teshuva mi’ahava, repentance out of love. This is when a person longs for closeness to his creator and regrets the evil that has severed the closeness of that relationship. This is the highest level of repentance. On the other hand, writes the Reishis Chochma, certain sins are so great that a lesser degree of repentance can not help. This lower repentance is called teshuva mi’yira, repentance out of fear (of punishment).

Finally, you are bothered why G-d punished Moshe after all of the good that Moshe had done (it should be noted that we do not know how much each mitzvah or sin is worth; one mitzvah could outweigh many sins, and vice-versa). I want you to know that the commentaries note that this is an extremely difficult chapter in the Torah. However, suffice it to say that Moshe repented fully and died without sin on his record. As to why G-d persisted in denying Moshe admittance into Israel, the Midrash seems to imply that this was not a punishment, and was rather an integral part of G-d’s larger plan.

I hope that this helps you,
Yochai Robkin