The Old Testament
The clearest statement here is in the book of Deuteronomy; it forms the classical statement on the unity of God.
"Hear, O Israel: The LORD our God, the LORD is one." (Deuteronomy 6:4)
This passage says more than that there is only one God. It tells us that the God of heaven is the only God, that he is one in every way. This statement is supported by many other passages in the Old Testament. Here are some more examples from the book of Deuteronomy:
"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)
"See now that I, even I, am he, and there is no god beside me;…" (Deuteronomy 32:39)
A particularly significant set of passages appears in Isaiah, where God reveals his nature as a challenge to those who follow other gods.
“'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) "Thus says the LORD, the King of Israel and his Redeemer, the LORD of hosts: 'I am the first and I am the last; besides me there is no god. Who is like me? Let him proclaim it. Let him declare and set it before me, since I appointed an ancient people. Let them declare what is to come, and what will happen. Fear not, nor be afraid; have I not told you from of old and declared it? And you are my witnesses! Is there a God besides me? There is no Rock; I know not any.'” (Isaiah 44:68)
"Thus says the LORD, your Redeemer, who formed you from the womb: 'I am the LORD, who made all things, who alone stretched out the heavens, who spread out the earth by myself,'" (Isaiah 44:24)
"I am the LORD, and there is no other, besides me there is no God;…" (Isaiah 45:5) "For thus says the LORD, who created the heavens (he is God!), who formed the earth and made it (he established it; he did not create it empty, he formed it to be inhabited!): 'I am the LORD, and there is no other.'" (Isaiah 45:18)
"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 Saviour; there is none besides me. Turn to me and be saved, all the ends of the earth! For I am God, and there is no other." (Isaiah 45:21,22)
"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)
These passages are absolutely clear. God is one, and the unity of God is one of the fundamental facts about his being.
This section of four chapters in Isaiah, which describes the nature of God in some detail, insists that God is one. This passage would have provided a perfect place to explain the idea of a threefold God, or of a God in more than one person, had such a picture been correct. The fact that this passage refers exclusively to the oneness of God is strong evidence that there is one God who is one in every way.
The last passage to this effect in the Old Testament is in the book of Malachi:
"Have we not all one Father? Has not one God created us?" (Malachi 2:10)
As a result of these, and many other passages, Jewish scholars from Old Testament times to the present day, including the period of the New Testament, have concluded that God is an absolute unity. There is one God, who is one person. The passage in Malachi refers to this God as Father.