In Parshat Chukat, the Torah records the fact that Moshe Rabbeinu was not permitted to enter the Land of Israel because of the mysterious sin he committed at the waters of Merivah two times. (In Parshas Pinchas it is mentioned another time and then again in Parshas Haazinu ).
1. “But the Lord said to Moshe and Aharon, ‘because you did not trust Me enough to affirm My sanctity in the sight of Bnei Yisrael, therefore you shall not lead this congregation into the land that I have given them.’ Those are the waters of Merivah [meaning] that Bnei Yisrael quarreled with the Lord through which He affirmed His sanctity.” (Bamidbar 20:12-13)
2. “At Har Chor, on the boundary of the land of Edom, the Lord said to Moshe and Aharon, ‘Let Aharon be gathered to his kin: he is not to enter the land that I have given to Bnei Yisrael, because you disobeyed my command about the waters of Merivah.’” (Bamidbar 20:23-24)
3. “The Lord said to Moshe, ‘Ascend these heights of Avarim and view the land that I have given to Bnei Yisrael. When you have seen it, you too shall be gathered to your kin, just as your brother Aharon was. For, in the wilderness of Zin, when the community was contentious, you disobeyed My command to uphold My sanctity in their sight by means of the water.’ These are the Waters of Merivat-kadesh, in the wilderness of Zin.” (Bamidbar 27:12-14)
4. “That very day the Lord spoke to Moshe: ‘Ascend these heights of Avarim to Har Nevo, which is in the land of Moav facing Yericho, and view the land of Canaan which I am giving Bnei Yisrael as their holding. You shall die on the mountain that you are about to ascend, and shall be gathered to your kin, as your brother Aharon died on Har Chor and was gathered to his kin; for you both broke faith with Me among Bnei Yisrael, at the waters of Merivat-kadesh in the wilderness of Zin, by failing to uphold My sanctity among the Israelite people.’” (Devarim 32:48-51)
The Rambam, in the fourth perek of Shemoneh Perakim, his introduction to his commentary on Pirkei Avot, discusses Moshe’s sin in the context of his admonition never to leave the “middle way.” The Rambam writes there how the mitzvot of the Torah in general act as a discipline for attaining the moral virtues of moderation, generosity, contentment, gentleness and modesty. Moshes’ sin was the deviation from the middle path, when he became angry with Bnei Yisrael.
In her Iyyunim Hadashim ‘al Sefer Ba-Midbar, pp. 246ff., Nechama Leibowitz writes how one can discern in the Rambam’s remarks two specific subcomponents of the sin of Moshe. First, he committed a personal sin: he departed from the middle path and tilted towards the vice of anger. But it is hard to understand how this by itself was such a heinous crime. The Rambam, therefore, adds a second component: by getting angry, Moshe misled the people as to the nature of God. The masses felt that Moshes’ anger was reflective of God’s anger. The masses therefore thought that the All-merciful God was in essence a wrathful deity.
According to this approach, Moshes’ sin was not just one of middot. It was also one of de’ot (correct doctrinal beliefs that a Jew must possess). I think that this approach is parallel with the Rambam’s understanding of Miriam’s sin of the gossip against Moshe, depicted in Parshat Behalotecha as well. In that case the Rambam also emphasizes that the punishment of leprosy that Miriam incurred was not the sin of middot per se that (as important as middot are in Judaism!). It was the sin of de’ot, in that case a false equation of the spiritual level attained by Moshe in his prophecy, with that of other prophets. In reality, the prophetic level of Moshe was sui generis.
This notion can be connected to another feature of the Rambam’s understanding of biblical descriptions of God. The Rambam believes that not only anthropomorphic descriptions of God are false, but anthropopathic descriptions as well. That is, not only is it incorrect to say “God possesses hands, fingers,” etc., for only humans possess bodies, and not God, it is also incorrect to say that God possesses human emotions or feelings, such as anger, jealousy, etc. Consequently, any biblical descriptions of God that contain descriptions of emotions are only to be understood as allegories, but do not describe anything that really exists. (According to the Rambam, what we say every day in the second paragraph of Shema [vecharah af Hashem bachem] is not a literal description of God’s response but an allegorical description!) If Klal Yisrael were led to understand that God is “really angry,” they would possess a theologically incorrect notion of the incorporeal God that does exist. On the other hand, according to the Rambam, although the middot of rachum vechanun are not essential positive attributes of God (according to the Rambam, the essence of God can only be described negatively, i.e., what He is not), the terms rachum vechanun do represent God’s attributes of action in this world (See Moreh Nevuchim, I: 54). And this world is tov me’od; it is a world expressive of God’s actions of rachum vechanun.