Why do we say God will be one when we also say He already is?

Question

Aleinu ends with the phrase "on that day God will be one and His name will be one". While we could interpret this to mean that "on that day everybody else will agree with us on this", the plain meaning of the text is more direct. Yet we affirm, multiple times a day, that God is already one.

Why was this text chosen for Aleinu instead of some other text that better expresses the intention? And is the intention in fact that God will be seen as one by everybody, or is this text meant to convey something else?

Answer 1

When we say in the shema, "ה' אחד", we mean that God is one in the fact that he is indivisible in such a way that is unimaginable. (See Mishna Torah, Hil. Yesodei Hatorah 1:7.)

Here, the intention is not the oneness of God as an entity, but rather God's uniqueness in that he is the only "godly" being and that no other "gods" have such power. So the verse "ביום ההוא יהיה ה' אחד ושמו אחד", means that eventually, God and his name will be one, i.e. He will be recognized as the only God and will be served by all the nations as such. (See Abarbanel, Zecharia 14:9.)

Why do we pick this verse to say at the end of Alenu and not others? Well, we are trying to use this verse as a source for the fact that not only will all the nations eventually serve God as their king(ויקבלו כלם את עול מלכותיך), but also that all other gods will be worthless and destroyed and forgotten (לְהַעֲבִיר גִלּוּלִים מִן הָאָרֶץ וְהָאֱלִילִים כָרוֹת יִכָרֵתוּן). The only other p'sukim I can think of that relate the nations' eventual serving of God (Tzefania 3:9; Yeshayahu 2:2) don't really specify that God will be unique in their serving him; they don't specify that the nations won't still be serving their old gods as well. (Ch. 2 in Yeshayahu does bring this out, but it takes several p'sukim to get the message across, and it also doesn't emphasize God's kingship, unlike in Zecharia where it is all nice and compact in this one pasuk.)

This verse: "ביום ההוא יהיה ה' אחד ושמו אחד", as it was explained above, brings across that very message that God will be served by all the nations as the one true God.

[Note that the gemara (Pesachim 50a) asks your first question about God's oneness. The answer it gives (על דרך דרש) is that today God's name is spelled "ykvk" and pronounced "adonai". In the Messianic era, His name will be pronounced the way it is spelled.]

Answer 2

(This is in answer to your second question, what the text means, not your first, why it was chosen.) The text is from Z'charya 14:9. The commentaries there explain that it means that God will be seen as one by everyone.

Answer 3

Clarifying all the above -- the emphasis is not so much on G-d being One, but then, also, "His name will be One". Name is based on external perception; all will regard Him as such.