Carmel: I guess Zoroaster, although never mentioned in the O.T., was indirectly instrumental in Yhwh's becoming God. After all, most of the OT was written or rewritten under Persian rule. Only over against Zoroastrian dualism the first and last emergence of real monotheism in Second Isaiah makes sense (45:7):
I form light and create darkness,
I make weal and create woe;
I Yhwh do all these things.
I wrote "first and last", because Jewish monotheism soon fell back into practical dualism in the Persian mood, with the constitution of Satan as an anti-God (very similar to the Persian Angra Mainyu), culminating in the radical opposition of the "spirit of light" and the "spirit of darkness" in the Qumran texts (actually, the previous Persian innovation appeared as defensive orthodoxy under the new cultural threat of Hellenism).
Aside from that, I agree that a very large part of the world remained "God-free" to this very day, and this remains the biggest challenge to monotheism.