Free translation from a talk of the Rebbe, 5th Night of Sukkot, 5745 (1984),(excerpt)

The principal Ushpizin of the fifth day of Sukkos is Aharon, the priest. His counterpart among the ‘Chassidic Ushpizin’ is the Tzemach Tzedek. They both share a common quality — the service of Ahavas Yisrael, the establishment of love and unity among the Jewish people. Aharon is known for that quality, as the Mishnah declares: ‘Be of the disciples of Aharon, loving peace and pursuing peace, loving your fellow creatures, and bringing them near to the Torah.’ Indeed, the Midrash elaborates on the extensive efforts Aharon would make to establish peace and harmony among the Jewish people.

Similarly, it was in the era of the Tzemach Tzedek that unity was restored to the Jewish people. In the previous generations, there were traces of the bitter rift between Misnagdim and Chassidim. However, the Tzemach Tzedek, through extensive meetings with the leaders of all factions, removed all the obstacles that stood in the way of unity. Just as a great light draws to it all the sparks, the Tzemach Tzedek was able to attract and eventually bring about cooperation and joint activity from those groups who were removed from Chassidim. Furthermore, the unity established in that generation set the spirit for cooperation and unity in future generations, thus, establishing oneness among the Jewish people.

The oneness of the Jewish people and love for one’s fellow Jews are principles that are constantly relevant and important. However, on this day, these qualities receive even greater emphasis and stress.

3. Though, as mentioned above, the corresponding traditional and Chassidic Ushpizin in this case, Aharon the priest, and the Tzemach Tzedek, share a common quality, nevertheless, as explained in the sichos of the previous nights, there is a difference and even a direct contrast between the two. The contrast complements and expresses the unique nature of each quality of the Ushpizin, the Chassidic Ushpizin being the inner aspect of the traditional Ushpizin. To fully appreciate the lessons we can learn from them, we must explore not only their similar qualities, but also those dimensions in which they revealed different traits and natures. Indeed, through discovering a common quality between men of different characters, a greater dimension is revealed, as explained in Chassidic thought regarding the fusion of the attributes of Chessed and Gevurah in the attribute of Tiferes.

Thus Aharon is distinguished by the fact that he exercised the power of love, reaching out to Jews of a low level as the Mishnah emphasizes ‘loving creatures’; that term referring to those individuals who possess no other redeeming characteristic other than the fact that they were created by G‑d. Aharon would reach out to these individuals and make an effort to establish peace between them, even lying if necessary. In contrast, the Tzemach Tzedek’s efforts in establishing Jewish unity were carried out, to a large extent, among Torah leaders. Though he also was involved with common people as emphasized by the popularly known story in which he gave a loan to a butcher before prayer, the efforts towards Jewish unity for which he is known are those with the leaders of our people. Furthermore, the manner in which he established Jewish unity and overcame the obstacles separating the Misnagdim from the Chassidim was by teaching Torah. For example, one of the major differences of opinion between the Misnagdim and Chassidim was the Alter Rebbe’s conception of the Tzimtzum. The Alter Rebbe wrote that the Tzimtzum cannot be understood in a simple sense, and thus, G‑d’s presence is found in this world, as well. In the Alter Rebbe’s time, this doctrine had opponents. However, in the Tzemach Tzedek’s time, even R. Chaim Volozin, the Vilna Gaon’s major disciple, writes that the Tzimtzum cannot be understood simply. This was the manner in which the Tzemach Tzedek established Jewish unity. Furthermore, his efforts brought about cooperation even in regard to establishing government policy.

Thus, the combination of the activities of Aharon and the Tzemach Tzedek demonstrate how our efforts to establish Jewish unity must be directed to individuals on every level from the highest rungs — the leaders of our people, to the lowest levels — ‘the creatures.’ The yetzer hora (evil inclination) may attempt to sway an individual from these two services. In regard to the ‘creatures’ — it argues, ‘why should you descend to their level? Let someone else do it.’ Similarly, in regard to the Torah leaders, it may argue: ‘Who are you to talk to such greats? Don’t you know your own humble status?’ Furthermore, the yetzer hora may argue: ‘Why is it so necessary to bring a Torah leader to true and complete love of the Jewish people, he has many other fine qualities?’ From the example of these two greats, we see how efforts must be made to reach out to all Jews.

Even a Torah giant needs the aspect of Ahavas Yisrael. On the contrary, if he is lacking in this quality, his entire service is blemished. Furthermore, because he is a Torah giant, that blemish is more noticeable. The Talmud teaches that a Torah scholar who wears soiled clothing should be punished by death; i.e. because he is a Torah scholar, his punishment is severe. Similarly, when a Torah scholar lacks Ahavas Yisrael, that deficiency is serious.

To return to the concept of oneness, our sages write, ‘peace is great. Even when the Jews worship idols, if there is peace among them, G‑d says, their enemies will not rule over them.’ Similarly, the Talmud declares that the generation of Achav were all idol worshipers. Nevertheless, because there was no strife between them they would go out to war and be victorious. In contrast, the generation of Dovid were Torah sages of the highest degree. Nevertheless, they would fall in battle because of internal strife. Thus, we see how Ahavas Yisrael is not merely an added merit, but a quality that is essential to a Jew’s very existence.

The above can also be related to the Torah portion of the week, the second portion of VeZos HaBerachah: That portion describes the service of the tribe of Levi as the verse declares ‘they shall teach Your judgments to Yaakov and Your Torah to Israel.’ Though this portion describes the service of one particular tribe, it is relevant for the entire Jewish people. The Rambam writes that any Jew who makes a commitment to give himself over to Torah and mitzvos is, in a spiritual sense, ‘a Levi.’

The Levi’im are representatives of Jewish unity. Indeed, the very name Levi was given by Leah, ‘for now my husband will be united with me.’ In an ultimate sense, this verse refers to the unity of the Jewish people with G‑d. That unity will be revealed in the Messianic age, when all the Jews will leave the exile; not one Jew will remain. In the redemption from the Babylonian exile, many Jews, among them the Torah sages and the Levi’im, remained in Babylon. However, in regard to the future redemption ‘a great congregation will return,’ ‘you, children of Israel, will be gathered one by one,’ no Jew will remain in exile. Thus, in preparation for this redemption, efforts must be made to increase Jewish unity as explained above in regard to the Ushpizin related to the present evening.