There is a continuous body of teaching in both the Old and New Testaments relating to the Tri-unity of God.
Rabbis who believed that each word of the Hebrew Scriptures, each letter, is God’s revelation had to admit that God spoke to himself and referred to himself in the plural. How can that be, when we know there is only one God?
Whatever else may be in question, we all agree that in the Jewish religion there is only one God. Yet there is disagreement as to the origin of Jewish monotheism.