It seems upon course consideration that God has always taught that he is the only actual God. But upon closer inspection, it appears that God reveals himself to Abraham, Isaac, and Jacob as "El Shaddai" which is "God Almighty" or "The Almighty God", El being the general word for "a god" in the Canaanite language. He is also referred to as "El Elyon" which is "Most High God" or "Uppermost God."
He says to Moses in Exodus 6:3,
"I appeared to Abraham, Isaac, and Jacob, as God Almighty, but by My name, YHWH, I did not make Myself known to them."
These names from Genesis, as well as much of the dialogue in the narrative that forms a context for them, seem to me to suggest that the patriarchs revered YHWH as the greatest of the gods or the god of gods, but not necessarily the only real God.
Later, the Spirit of Prophecy reveals through Isaiah that YHWH is the only true God.
Isaiah 43:9-13 (NASB)
9 All the nations have gathered together
So that the peoples may be assembled.
Who among them can declare this
And proclaim to us the former things?
Let them present their witnesses that they may be justified,
Or let them hear and say, “It is true.”
10 “You are My witnesses,” declares the Lord,
“And My servant whom I have chosen, So that you may know and believe Me
And understand that I am He.
Before Me there was no God formed,
And there will be none after Me.
11 “I, even I, am the Lord,
And there is no savior besides Me.
12 “It is I who have declared and saved and proclaimed,
And there was no strange god among you;
So you are My witnesses,” declares the Lord,
“And I am God.
13 “Even from eternity I am He,
And there is none who can deliver out of My hand;
I act and who can reverse it?”
Isaiah 44:6 Thus says the LORD, the King of Israel and his Redeemer, the LORD of hosts: 'I am the first and I am the last, And there is no God besides Me.
I can't find any references before Isaiah that clearly state that there are no other gods, without qualifiers like "before me" or "like me". Where in the biblical narrative is it first made clear that YHWH is not only the most powerful and greatest God, but he is the One True God, and that all other beings or objects that might be called "gods" are false?