I was surprised to find out that the earliest explicit confession of monotheism in the Bible is Isaiah 44:6: "I am the first and the last; besides me there is no God." In case we missed it he repeats his confession twice in 45: 5-6: "I am the Lord, and there is no other; besides me there is no god....there is no one besides me; I am the Lord and there is no other."
The author of the second part of Isaiah, chapters 40:1-55:13, was living in Babylon during the Exile about 540 years before Jesus. He probably did not arrive at this revolutionary insight all by himself. No doubt, he and his fellow Jewish thinkers in exile arrived at this conclusion as they looked askance at the many "gods" of the Babylonians. The remarkable insight that there is only one God is so mind-blowing that it colors our whole way of reading the Old Testament even before Isaiah.