The haftarah of Shabbos Shuvah comes from Hoshe'a. Chazal famously comment:
אמר לו הקב״ה לראובן מעולם לא חטא אדם לפני ועשה תשובה ואתה פתחת בתשובה תחלה חייך שבן בנך הושע פותח בתשובה שנאמר שובה ישראל.
Yet we know that there were in fact others who did teshuvah before Reuvain. Kayin did teshuvah. Adam did teshuvah. Why do Chazal single out the teshuvah of Reuvain as being the first?
What in fact Reuvain's? Braishis 35:22 tells us:
וַיְהִ֗י בִּשְׁכֹּ֤ן יִשְׂרָאֵל֙ בָּאָ֣רֶץ הַהִ֔וא וַיֵּ֣לֶךְ רְאוּבֵ֗֔ן וַיִּשְׁכַּ֕ב֙ אֶת־בִּלְהָ֖ה֙ פִּילֶ֣גֶשׁ אָבִ֑֔יו וַיִּשְׁמַ֖ע יִשְׂרָאֵֽ֑ל
yet that very same pasuk ends
וַיִּֽהְי֥וּ בְנֵֽי־יַעֲקֹ֖ב שְׁנֵ֥ים עָשָֽׂר
upon which Rashi comments
ורבותינו דרשו (בבלי שבת נ״ה:): ללמדנו בא, שכולם שוים וכולן צדיקים, שלא חטא ראובן.
The Yismach Yisrael of Aleksander writes that this is why Reuvain is held up as the paragon of teshuvah. He did something wrong, but immediately regretted and repented on his actions, such that there is not even a pause between the sin and the tikun of וַיִּֽהְי֥וּ בְנֵֽי־יַעֲקֹ֖ב שְׁנֵ֥ים עָשָֽׂר. The same cannot be said of Adam and Kayin.
That being said, just a few parshiyos later, when we read about the sale of Yosef, the Torah relates how Reuvain tried to save his brother, but then vanishes from the story while the brothers go and sell Yosef into slavery. Reuvain comes back to retrieve Yosef and discovers him missing (37:29):
וַיָּ֤שׇׁב רְאוּבֵן֙ אֶל־הַבּ֔וֹר וְהִנֵּ֥ה אֵין־יוֹסֵ֖ף בַּבּ֑וֹר
Where was Reuvain while the sale was going on?
Rashi tells us: דבר אחר: עסוק היה בשקו ותעניתו על בילבול יצועי אביו.
You can't have it both ways! If Reuvain did nothing wrong, or at least immediately repented for whatever slight mistake he made, then why here is he doing teshuvah again? Why is this moment the time the rehash old wounds?
I think our parsha contains the answer. In pesukim 10-13 of our parsha we have the mitzvah of hakhel, which, as we've discussed in the past, really would seem to fit better in parshas Shoftim, where we have the laws that pertain to a king, or maybe in the parshiyos of the moadim, since hakhel takes place on Sukkos. Following that mitzvah we have pesukim which portend sin and galus:
וְחָרָ֣ה אַפִּ֣י ב֣וֹ בַיּוֹם־הַ֠ה֠וּא וַעֲזַבְתִּ֞ים וְהִסְתַּרְתִּ֨י פָנַ֤י מֵהֶם֙ וְהָיָ֣ה לֶאֱכֹ֔ל וּמְצָאֻ֛הוּ רָע֥וֹת רַבּ֖וֹת וְצָר֑וֹת׃
It sounds like Bnei Yisrael repent:
וְאָמַר֙ בַּיּ֣וֹם הַה֔וּא הֲלֹ֗א עַ֣ל כִּי־אֵ֤ין אֱלֹקי֙ בְּקִרְבִּ֔י מְצָא֖וּנִי הָרָע֥וֹת הָאֵֽלֶּה׃
But instead of acceptance and forgiveness, the next pasuk (18) follows with a promise of greater hester panim:
וְאָנֹכִ֗י הַסְתֵּ֨ר אַסְתִּ֤יר פָּנַי֙ בַּיּ֣וֹם הַה֔וּא עַ֥ל כׇּל־הָרָעָ֖ה אֲשֶׁ֣ר עָשָׂ֑ה כִּ֣י פָנָ֔ה אֶל־אֱלֹהִ֖ים אֲחֵרִֽים
Ramban and everyone asks why it is that Hashem responds to this overture of repentance by hiding?
R' Simcha Bunim Sofer gives a magnificent answer. He writes that the bris we read about in Nitzavim created the idea of arvus, the idea that no one in Klal Yisrael is an island. One person's wrongdoing bears not just on the individual, but weighs down the nation as a whole.
The statement עַ֣ל כִּי־אֵ֤ין אֱלֹקי֙ בְּקִרְבִּ֔י is all about personal wrongdoing, personal repentance. "B'kirbi" -- it's all about me. This type of repentance misses the mark because there is no acknowledgment of belonging to something greater, of having caused harm to something greater.
Perhaps that is why the Torah deliberately uses the mitzvah of hakhel as an introduction to this parsha. Hakhel, the gathering together of men, women, even little children, as one, is the antithesis of the selfish teshuvah where the individual is concerned only with his/her own spiritual plight to the exclusion of others.
Returning to Reuvain, וַיִּֽהְי֥וּ בְנֵֽי־יַעֲקֹ֖ב שְׁנֵ֥ים עָשָֽׂר certainly proves that Reuvain was in good standing, no different than his brothers, but at the same time, this same pasuk underscores the fact that these were 12 separate individual sons, not one united family. Reuvain may have regretted moving his father's bed, but his actions opened a Pandora's box and exposed the underlying feeling that not all Yaakov's wives, and by extension not all the brothers, were in fact equal, and hence not united as one. The result that would play out over time would be the sale of Yosef.
This leads to the second teshuvah of Reuvain, the teshuvah he comes to when he sees the plot against Yosef playing itself out. This is not his repentance as an individual for the sin of moving his father's bed, but repentance for the effect of his sin on the community, on the family structure. This is the repentance of arvus, of feeling for the plight of his brother.
Adam repented, Kayin also repented. Reuvain had already done his own private repentance as well. But what we celebrate on Shabbos Shuvah is not just repentance for private wrongdoing, but the repentance for the harms done to the nation, to the community, by our own shortcomings.