One opinion in the Medrash (Bereishis Rabbah 15:7) maintains that the forbidden fruit from which Adam ate in the Gan Eden was an esrog. If so, how are we able to use an esrog as one of the four species on Succos (Vayikra 23:42), when there is a Talmudic principle (Rosh Hashanah 26a) that אין קטגור נעשה סניגור – an object that was used for a sin may not be used in to perform a mitzvah?
In the introduction to his sefer Shu”t Bais Ephraim, Rav Ephraim Zalman Margulies explains that the esrog teaches us the tremendous power of teshuvah. Because Adam did teshuvah for his sin, the very object that he used for his transgression was transformed into a mitzvah that brings great joy. This illustrates the idea expressed by the Rambam (Hilchos Teshuvah 7:6) that before a sinner repents, he is disgusting to Hashem, but after he does teshuvah, he once again becomes beloved.
In the tefillah for rain recited by the chazzan during his repetition of tefillas Mussaf on Shemini Atzeres, each stanza invokes the water-related merits of one of our righteous forefathers. In the stanza referring to Moshe, we mention the fact that when the Jewish people were thirsty, he struck the rock and caused water to come forth, and we daven that in the merit of his righteousness, Hashem should bless our water supply.
Since Moshe was punished for his actions and was not allowed to enter Eretz Yisroel as a result, why do we invoke an episode that is considered more of a sin than a merit?
Rabbi Reisman suggests that because Succos comes just after we have done teshuvah and been forgiven for our sins on Yom Kippur, therefore it is a time when we do not need to fear prior misdeeds and specifically invoke them, as our teshuvah transforms them into merits.
Along these lines, Rav Moshe Aharon Friedman points out that on Rosh Hashanah, we symbolically cast our transgressions into water during Tashlich, while the tremendous joy of Succos revolved around the drawing of water for libations on the mizbayach. Although we threw our sins into the water just a short time earlier, after we have done teshuvah out of love and turned our misdeeds into merits, we return to the water and metaphorically retrieve the aveiros that we recently placed there, confident that they have now been elevated to the extent that they are now fit for the mizbayach.
As we enter Succos fresh off the spiritual high of Rosh Hashanah and Yom Kippur, our closeness to Hashem should continue as we rejoice and celebrate the uplifting feeling of being baalei teshuvah whose repentance was accepted. (R’ Ozer Alport)
