It is complicated. There was no monotheist Judaism until fairly late.

I won't speak directly to the question but I will give some background on what preceded the development of monotheism. Setting the stage, so to speak.

Briefly, what we know as monotheist Judaism had its origins in the Canaanite pantheon headed by El, and including Baal, Yahweh, and Asherah. Over many centuries, during which the people who would become Israrlites, two major identities developed. The northern kingdom (Israel) held somewhat close to the Canaanite practices of worship, with Baal playing a major role, and maintaining the cultic places and practices. In the southern kingdom (Judah) they diverged quite a bit from their northern brethren. Through what Mark Smith calls "convergence and differentiation" the southern peoples merged things up, changed some gods roles, and so on. Asherah (probably) dropped out as a distinct deity. Baal was deprecated. Yahweh became the chief.

Around the sixth century BCE there was a lot of political stuff going on, the important things here being the collapse of the Assyrian empire and resulting diminishment of the power of their client state, Israel. Judah's King Josiah's priests "found" some ancient scrolls which were used as a theopolitical tool to assert the dominance of Judah. Sort of "not only do worship the right deity in the right way, we have always worshiped the right deity in the right way." Josiah enforced monolatry, the state religion of the southern kingdom, on both kingdoms.

Monotheistic Judaism emerged over the next few centuries. How and why that happened takes a lot more to explain, nor am I well qualified to speak to it.

Follow up question: OP asserts that everyone around them was polytheist but I was under the impression that Zoroastrianism and the sun-worship of Akhenanten were considered proto-monothesitic.

I remember long ago reading something about the possibility of Akhenaton in fact being Moses. Anybody have any info on that speculation?