There were monotheists before the Babylon incident, but Ezra the scribe and the elites who were exiled to Babylon reinvented it to suit their own purposes, mainly increasing their power and wealth and purging the goyim from the "holy race". And one of the things they did was rewrite their version of God to make him more appealing to the pagans, which is why "God" in the Torah and some of the legend books is so radically different than in the prophets and most of the psalms and wisdom books, and the New Testament. Bear in mind that the Torah is probably the newest addition to the Old Testament besides Ezra-Nehemia, the ones who most likely wrote it. And they combined pretty much every tradition they could find to appease everyone, which is why the Torah contradicts itself all the time and repeats itself so much.

What's interesting is that until recently Jews and Christians actually admitted that Josiah and Ezra wrote the Law books. Their excuses were that Josiah "found" Deuteronomy in a temple after it had somehow been completely forgotten and lost everywhere else (around the time of Jeremiah, which has several complaints about scribes writing false books), and Ezra had to "rewrite" the rest of it because apparently some king burned it, as if they would only have one copy of such a holy text.