“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.(Isa. 43:10)

The Jews were surrounded by polytheist. This is a polemical statement to the ideas of the polytheist. In ANE worldviews the gods are byproducts of other gods and eventually the universe. The Biblical picture is a God that is the grounds for all reality:

The gods of the pagan world were often spoken of as formed, created, coming into being or standing in genealogical relationship to each other. The Lord proceeds from none and is succeeded by none, existing from the beginning, without superior or inferior, one only God.

Motyer, J. Alec. The Prophecy of Isaiah: An Introduction & Commentary (Kindle Locations 9640-9641). InterVarsity Press. Kindle Edition.