Zechariah 14:9

וְהָיָה ה' לְמֶלֶךְ עַל כָּל הָאָרֶץ בַּיּוֹם הַהוּא יִהְיֶה ה' אֶחָד וּשְׁמוֹ אֶחָד

Hashem will be King over all the earth; on that day Hashem will be One, and His Name One.

This verse is familiar not only from the Rosh Hashana Malkiyos, but also as the last line of the prayer Aleinu. Rashi explains that G-d “will be One” in that the other nations will abandon foreign forms of worship and recognize that G-d alone rules all. Furthermore, His Name will be One in that His Name alone is the one that will be expressed by all mankind.