Your question is only partially correct in its premise. It would be more accurate to say that YHWH was the god settled on by the ancient Jews.

The question of how exactly YHWH got to the status of monotheistic deity is one of the great questions in the history of ideas. Almost every monotheistic tradition in the world dates back to this development in one way or another, suggesting that it was a fairly unusual event, and the circumstances that brought it about were (more or less) unique.

The Yahwistic tradition has murky historical roots, but most sources agree that it has its roots in the desert. Its earliest adherents were nomads, slaves, outcasts, wandering tradespeople, and generally those outside the power structures of society. The word Hebrew itself probably originally means something like “migrant laborer.” (The Egyptian language referred to such people as “Shasu” and spoke of their god, YHW).

At some point around the year 1200 and the Bronze Age Collapse, the highlands of the Levant filled with small, non-hierarchically organized settlements. The people who lived here eventually developed a cultural identity as being the sons of “Israel.” They were ruled by tribal structures, military heroes who rose up from time to time, and a variety of religious figures. Yahweh seems to have been venerated (though not by all; see Judges 21:19, where Yahwists are seen as different and exotic). The center of Yahwistic activity was in Shiloh, though there were itinerant Yahwists who called themselves “Levites” throughout the land, often marked by Egyptian names and claiming descent from escaped Egyptian slaves.

Eventually, these disparate tribes unified into a single polity under a king, apparently in response to foreign threat. (1 Samuel 8-9) Yahwists seem to have had some involvement in this process, others protested the movement toward kingship, instead recommending a contract/covenant directly between the people and their god, Yahweh. Yahweh was distinguished in this age by being extremely concerned with Justice, but with a great emphasis on “Fairness” and fairly little emphasis on “Order.” Order was nice and all, but where earthly systems violated the rights of the powerless, they were to be discarded or destroyed.

While all the other gods worshipped in Israel seem to have been worshipped elsewhere under slightly different rites, Yahweh remained unique. Yahweh seems to have been regarded as the outcast god of an outcast people.

Enter David. Myth and history are tangled with David, but a credible historical picture is sketched by the extant evidence: David led a rebel band and eventually seized the kingship, probably with Philistine aid. He did so with some measure of popular support, galvanized by enthusiastic support of the Yahwistic prophets and priests, who were attacked violently by King Saul for their support of the revolution. David established a new state and the events of his reign guaranteed a permanent place for the Yahwistic prophets and priesthood (who were not infrequently at odds with each other; the radical ethical emphasis of the prophets seems to have won the argument in the end).

But Yahwism sat ill with state power. The outcast origins and prophetic critiques of the accumulation of money and power were basically anti-authority. Kings tried with varying degrees of success to control the Yahwists, or else to displace their power by favoring other cults. They were ultimately unsuccessful in this attempt, though the experience of religious competition probably increased the anti-pantheon tendencies inherent in Yahwism.

Eventually, the Judahite and Israelite kingdoms (which split shortly after the initial unification) were overthrown by Mesopotamian Empires. It seems to have been the experience of Exile that caused the full shift to what we would call monotheism. State power vanished, priestly power vanished, but the prophets endured, and won over the hearts and minds of the people who now called themselves “Jews.” When the state was reconstituted, priests of Yahweh controlled all levers of power, compiled the traditions of their religion into what we call the Hebrew Bible, and attempted a several-centuries long experiment that resulted in the environment that produced the ground on which Christianity and Judaism were born in the first century CE.

Primary Sources:

Psalm 82 (YHWH threatens to usurp and kill the children of El for their failure to protect the weak)

1 Kings 18 (A contest of gods that mirrors the contest between the people and prophets against state power)

Isaiah 40-55 (Second Isaiah; very clear monotheist shift in evidence here)

Secondary Sources:

-Thomas Römer, The Invention of God (Cambridge: Harvard Univeristy Press, 2015). (Good overview text.)

-Mark S. Smith, The Origins of Biblical Monotheism (Oxford: Oxford University Press, 2001). (Compare to above.)

-Seth Sanders, From People to Public in the Iron Age Levant. (For the development of the modern idea of a representative polity through emerging monotheism).

Did the idea of a messiah help with the religion in any way?