God is absolute unity, and we, His people, are also required to express this oneness, through behavior that demonstrates that we are one nation.

“You shall love your fellow as yourself” (Leviticus 19:18), for this is “a fundamental principle in the Torah” (Sifra, Kedoshim 4:12). This teaches us that it is not fitting for there to be division between one person and another; rather, we must judge each person favorably, as we would judge ourselves. This is [why the aforementioned verse ends with the phrase] “I am the Lord” (Leviticus 19:18); just as I, the Lord, am one, so too is it proper that you will be one, undivided nation. (Rav Avraham Saba, Tzeror HaMor, Leviticus 7)

Further reading: For more on loving one’s fellow Jews, see A Concise Guide to the Sages, p. 373.