The Hebrew Bible claims that the Jews as a people worshipped Yahweh exclusively from the time of the Covenant, albeit with the worship of “false” gods from neighboring lands sometimes undermining their unity (and inviting divine retribution on the part of Yahweh for those transgressions). There is no historical or archeological evidence that suggests a single unified religion in Israel or Judah during the period of the united Hebrew monarchy or post-Solomon split between Israel and Judah, however (the Hebrew Bible itself was written down centuries later). A more likely scenario is that the Hebrews, like every other culture in the ancient world, worshipped a variety of deities, with Yahweh in a place of particular importance and centrality. A comparable case would be that of the Assyrians, who emphasized the worship of Ashur but who acknowledged the existence of other gods (including Yahweh).
As the Hebrews became more powerful, however, their religion changed dramatically. A tradition of prophets, later remembered as the Prophetic Movement, arose among certain people who sought to represent the poorer and more beleaguered member of the community, calling for a return to the more communal and egalitarian society of the past. The Prophetic Movement claimed that the Hebrews should worship Yahweh exclusively, and that Yahweh had a special relationship with the Hebrews that set Him apart as a God and them apart as a people. The Prophetic Movement lasted from the period before the Assyrian invasion of Israel through the Babylonian Captivity of the Jews, from about 750 BCE - 550 BCE.
This new set of beliefs, regarding the special relationship of a single God to the Hebrews, is referred to historically as the Yahwist religion. It was not yet "Judaism," since it did not yet disavow the belief that other gods might exist, nor did it include all of the rituals and traditions associated with later Judaism. Initially, most of the Hebrews continued to at least acknowledge the existence of other gods – this phenomenon is called henotheism, the term for the worship of only one god in the context of believing in the existence of more than one god (i.e. many gods exist, but we only worship one of them). Over time, this changed into true monotheism: the belief that there is only one god, and that all other “gods” are illusory.
The Prophetic Movement attacked both polytheism and the Yahwist establishment centered on the Temple of Jerusalem (they blamed the latter for ignoring the plight of the common people and the poor). The prophets were hostile to both the political power structure and to deviation from the exclusive worship of Yahweh. The prophets were also responsible for enunciating the idea that Yahweh was the only god, in part in reaction to the demands of Assyria that all subjects acknowledge the Assyrian god Ashur as the supreme god. In other words, the claim of the Prophetic Movement was not only that Yahweh was superior to Ashur, but that Ashur was not really a god in the first place.
This is, so far as historians know, the first instance in world history in which the idea of a single all-powerful deity emerged among any people, anywhere (although some scholars consider Akhenaten’s attempted religious revolution in Egypt a quasi-monotheism). Up to this point, all religions held that there were many gods or spirits and that they had some kind of direct, concrete connections to specific areas. Likewise, the gods in most religions were largely indifferent to the actions of individuals so long as the proper prayers were recited and rituals performed. Ethical conduct did not have much influence on the gods (“ethical conduct” itself, of course, differing greatly from culture to culture), what mattered was that the gods were adequately appeased.
In contrast, early Judaism developed the belief that Yahweh was deeply invested in the actions of His chosen people both as a group and as individuals, regardless of their social station. There are various stories in which Yahweh judged people, even the kings like David and Solomon, making it clear that all people were known to Yahweh and no one could escape His judgment. The key difference between this belief and the idea of divine anger in other ancient religions was that Yahweh only punished those who deserved it. He was not capricious and cruel like the Mesopotamian gods, for instance, nor flighty and given to bickering like the Greek gods.
The early vision of Yahweh present in the Yahwist faith was of a powerful but not all-powerful being whose authority and power was focused on the Hebrew people and to the territory of the Hebrew kingdom only. In other words, the priests of Yahweh did not claim that he ruled over all people, everywhere, only that he was the correct God of the Hebrews and their land. That started to change when the Assyrians destroyed the northern kingdom of Israel in 722 BCE. Many of the Hebrews regarded this disaster as proof of the corruption of the rich and powerful and the righteousness of the Prophetic Movement. Even though the loss of Israel was an obvious blow against the Hebrews as a people, the worship of Yahweh as the exclusive god of the Hebrews gained considerable support in Judah. Likewise, as the exclusive worship of Yahweh grew in importance among the Jews (now sundered from the other Hebrews, who had been enslaved), the concept of Yahweh’s omnipotence and omnipresence grew as well.
The most important reforms of Hebrew religion occurred in the seventh century BCE. A Judean king, Josiah, oversaw the imposition of strict monotheism and the compilation of the first books of the Hebrew Bible, the Torah, in 621 BCE. In the process, the Yahwist priesthood added the book of Deuteronomy to older sacred writings (the priests claimed to have discovered Deuteronomy, but almost all historians of ancient religion believe that it was simply written at the time). When many Jews left the religion after Josiah's death, the prophet Jeremiah warned them that disaster would ensue, and when the Neo-Babylonians conquered Judah in 586 BCE, it seemed to validate his warning. Likewise, during the Babylonian Captivity, the prophet Ezekiel predicted the liberation of the Hebrews if they stuck to their faith, and they were indeed freed thanks to Cyrus (who admired older cultures like the Hebrews, since the Persians were originally semi-nomadic).
The sacred writings compiled during these events were all in the mode of the new monotheism. In these writings, Yahweh had always been there as the exclusive god of the Hebrew people and had promised them a land of abundance and peace (i.e. Israel) in return for their exclusive worship of Him. In these histories, the various defeats of the Hebrew people were explained by corruption from within, often the result of Hebrews straying from the Covenant and worshiping other gods.
These reforms were complete when the Neo-Babylonians conquered Judah in 586 BCE and enslaved tens of thousands of the Hebrews. The impact of this event was enormous, because it led to the belief that Yahweh could not be bound to a single place. He was no longer just the god of a single people in a single land, worshiped at a single temple, but instead became a boundless god, omnipotent and omnipresent. The special relationship between Him and the Hebrews remained, as did the promise of a kingdom of peace, but the Hebrews now held that He was available to them wherever they went and no matter what happened to them.
In Babylon itself, the thousands of Hebrews in exile not only arrived at this idea, but developed the strict set of religious customs, of marriage laws and ceremonies, of dietary laws (i.e. keeping a kosher diet), and the duty of all Hebrew men to study the sacred books, all in order to preserve their identity. Once the Torah was compiled as a single sacred text by the prophet Ezra, one of the official duties of the scholarly leaders of the Jewish community, the rabbis, was to carefully re-copy it, character by character, ensuring that it would stay the same no matter where the Jews went. The result was a “mobile tradition” of Judaism in which the Jews could travel anywhere and take their religion with them. This would become important in the future, when they were forcibly taken from Judah by the Romans and scattered across Europe and North Africa. The ability of the Jews to bring their religious tradition with them would allow them to survive as a distinct people despite ongoing persecution in the absence of a stable homeland.
Another important aspect of Judaism was its egalitarian ethical system. The radical element of Jewish religion, as well as the Jewish legal system that arose from it, the Talmud, was the idea that all Jews were equal before God, rather than certain among them having a closer relationship to God. This is the first time a truly egalitarian element enters into ethics; no other people had proposed the idea of the essential equality of all human beings (although some aspects of Egyptian religion came close). Of all the legacies of Judaism, this may be the most important, although it would take until the modern era for political movements to take up the idea of essential equality and translate them into a concrete social, legal, and political system.