Hear, O Israel, the L-rd is our G-d, the L-rd is One.

:שְׁמַע יִשְׂרָאֵל יהוה אֱלֹהֵינוּ יהוה אֶחָד

Sh'ma Yis-ra-eil, A-do-nai E-lo-hei-nu, A-do-nai E-chad.

Within Judaism there is no more a fundamental, foundational, basic, and uncompromising truth as ONE God. The oneness of G-d. Throughout the Torah G-d had said that he is The Lord, Whose Name is "Jealous One," is a jealous God.

Shemot - Exodus - Chapter 34

14 For you shall not prostrate yourself before another god, because the Lord, Whose Name is "Jealous One," is a jealous God.

To Judaism the idea of a godhead is too reminiscent of paganism such as the Babylonian three persons equilateral triangle. And The Egyptians were also notorious for using the triangle to denote the unity of a god consisted of three persons.   

Shemot - Exodus - Chapter 20

1 God spoke all these words, to respond:

2 "I am the Lord, your God, Who took you out of the land of Egypt, out of the house of bondage.

3 You shall not have the gods of others in My presence.

 

Mr. Apisdorf describes God's oneness as:

"If all of Judaism could be distilled into one statement it would be, 'Listen, O' Israel, God our Lord; God is One.' This is often understood as the proclamation of the Jewish belief in monotheism, one God. In fact, it is much more than that. It is a statement about the unity—the oneness—of God's absolute being. It's true that Judaism gave the world the single most revolutionary idea that has ever been articulated, namely, that there is only one God and not a myriad of conflicting and competing gods, forces, and powers. However, the oneness of God means even more than that. The oneness of God means that all that exists, all forces and all powers, are an expression of a greater, transcendent Unity that is the source of their existence as well as the source of any power or influence they seem to exert within existence" (Apisdorf, S. It All Begins With God.)

 

Source: Torah.org

Jewish author Tracey R. Rich, creator of the Judaism 101 website, encapsulates the oneness of God by stating:

"G-d is a unity. He is a single, whole, complete indivisible entity. He cannot be divided into parts or described by attributes. Any attempt to ascribe attributes to G-d is merely man's imperfect attempt to understand the infinite" (Source: Judaism 101.)

Understanding the ancient Israelites' conception of one God has to be OUR understanding of one God. Remember, the first five books of the Bible were written by Moses.