The Jews from the Beginning KNEW that there is a Plurality in Elohim - they did not know exactly who or what constituted the Plurality.
They speculated and, like today, they claim the angels are part of the plurality of Elohim.
But they are wrong and they cannot know or SEE their Messiah because their hearts are not circumcised and they are blinded having th eveil of Moses covering their minds.
The Jews to whom the Lord revealed Himself to came to know the Truth such as the prophet Isaiah and others.
Yahweh made it absolutely clear that He was the only God in the Old Testament. If you look at the words God and or Lord in the scriptures before they took His name out of the Old Testament it would say YHWH.
In the beginning, God created the heavens and the earth.
Genesis 1:1
“You are the Lord, you alone. You have made heaven, the heaven of heavens, with all their host, the earth and all that is on it, the seas and all that is in them; and you preserve all of them; and the host of heaven worships you.
Nehemiah 9:6
“You shall have no other gods before me.
Exodus 20:3
To you it was shown, that you might know that the Lord is God; there is no other besides him.
Deuteronomy 4:35
Know therefore today, and lay it to your heart, that the Lord is God in heaven above and on the earth beneath; there is no other.
Deuteronomy 4:39
Hear, O Israel: The Lord our God, the Lord is one.
Deuteronomy 6:4
“See now that I, even I, am he, and there is no god beside me; I kill and I make alive; I wound and I heal; and there is none that can deliver out of my hand.
Deuteronomy 32:39
And Hezekiah prayed before the Lord and said: “O Lord, the God of Israel, enthroned above the cherubim, you are the God, you alone, of all the kingdoms of the earth; you have made heaven and earth.
Isaiah 37:20
I am the Lord; that is my name; my glory I give to no other, nor my praise to carved idols.
Isaiah 42:8 (Of course they removed His name.)
“You are my witnesses,” declares the Lord, “and my servant whom I have chosen, that you may know and believe me and understand that I am he. Before me no god was formed, nor shall there be any after me.
Isaiah 43:10
I, I am the Lord, and besides me there is no savior.
Isaiah 43:11
I am the Lord, and there is no other, besides me there is no God; I equip you, though you do not know me,
Isaiah 45:5
Declare and present your case; let them take counsel together! Who told this long ago? Who declared it of old? Was it not I, the Lord? And there is no other god besides me, a righteous God and a Savior; there is none besides me.
Isaiah 45:21
Remember the former things of old; for I am God, and there is no other; I am God, and there is none like me,
Isaiah 46:9
That all the peoples of the earth may know that the Lord is God; there is no other. 1st Kings 8:60
There is none like you, O Lord, and there is no God besides you, according to all that we have heard with our ears. 1st Chronicles 17:20
Therefore you are great, O Lord God. For there is none like you, and there is no God besides you, according to all that we have heard with our ears. 2nd Samuel 7:22
And Hezekiah prayed before the Lord and said: “O Lord, the God of Israel, enthroned above the cherubim, you are the God, you alone, of all the kingdoms of the earth; you have made heaven and earth. 2nd Kings 19:15
But I am the Lord your God from the land of Egypt; you know no God but me, and besides me there is no savior.
Hosea 13:4
For you are great and do wondrous things; you alone are God. Psalm 86:10