The conception of the God of Israel presented in the Hebrew Bible seems to have developed out of two older polytheistic deities, El and Yahweh, who were worshipped in the Levant in the 2nd millennium BC.
The developed monotheism of later Judaism seems to have grown out of an earlier system of "monolatry", in which the God of Israel was worshipped exclusively but the existence of other deities was not denied. This monolatry in turn developed out of the original polytheism of the peoples of the Levant.
The decisive developments in the advent of monotheism appear to have been the rise of the Deuteronomic movement (7th and 6th centuries BC) and the Babylonian Exile (c.587-538 BC). Polytheistic ideas still remained current to some extent thereafter, however.
2. The original deities - El and Yahweh
The peoples of the ancient Levant were polytheists. Like the ancient Greeks, Romans and Egyptians, they believed in a pantheon of different gods and goddesses. The evidence is that these included a supreme god called El (Il), a powerful goddess called Asherah (Athirat), who seems to have been El’s wife, and the god Ba'al ("Lord"), who may or may not have been identical to the storm-god Hadad (Haddu). The gods of the Levantines overlapped with those worshipped in other parts of the ancient Near East: for example, Hadad equated with the Mesopotamian deity Adad.
The chief god El seems to have been one of the two predecessors of the God of the Bible. Indeed, "El" simply means "God" in Hebrew and other Semitic languages. In the Bible, the term often appears in the honorific form Elohim - grammatically, this is a plural form, but it generally seems to denote a single deity.
The second predecessor of the Biblical deity was a god called Yahweh or Yah. It is not entirely clear where Yahweh originated. Some scholars believe that he was originally a fully-fledged member of the Levantine pantheon of deities, and one theory would identify him with Yam, the god of the sea. However, his origins may well be more obscure. For example, he seems not to appear in the well-known archives of the Levantine city of Ugarit. He may originally have been a more marginal deity, worshipped in and around the region of Edom in the far south-east of the Levant. This possibility is indicated both by extra-biblical evidence and by fragments of early texts preserved in the Bible (Jdg. 5.4, Hab. 3.3). The Israelites themselves may have come from Edom, or at least lived there for a time. There is also evidence that he was originally a storm god (Jdg. 5.4, Ps. 29, 68.8-9).
Over time, the Israelites began to conceive the idea of a single national God, who was referred to with the names and attributes of El and Yahweh. The Israelites did not initially deny that other gods existed - they simply thought it improper to worship them (at least, some Israelites took this view: it was by no means universally held). This practice of worshipping a single god out of a pantheon is sometimes referred to as "monolatry".
A good example of the conflation of El and Yahweh into a single supreme God of Israel comes in Psalm 68, which may date from very early in Israelite history:
Sing to El, play music to his name…
in praise of him whose name is Yah….
Father of the orphans and judge of the widows is El in his holy dwelling....
El, when you went out before your people, when you strode through the desert,
the earth quaked and the heavens dripped,
before El, the God of the Sinai, before El, the God of Israel....
Mountain of El, Mount Bashan,
mountain of the humpbacked, Mount Bashan,
why did you lay in wait, mountain of the humpbacked,
mountain on which El desired to stay, yes, on which Yahweh wanted to dwell for ever?
However, another fragment of early poetry makes it clear that El-Yahweh existed alongside the old polytheistic gods:
El came from Teman,
the Holy One from Mount Paran....
Before him went Deber,
and Resheph followed close behind. (Hab. 3.3-5)
We can see an interesting shift over time. A number of scholars, including Jeffrey Tigay and Johannes de Moor, have analysed the composition of theophoric names found in the Bible and in inscriptions. Theophoric names are names containing the name of a deity, such as Ezekiel (-El) and Josiah (-Yah). In the period before the foundation of the Israelite monarchy (c.1000 BC), names referring to El are more plentiful than names referring to Yahweh, though there are a considerable number of Yahweh names too. In later periods, names referring to Yahweh become increasingly predominant, peaking around the end of the 7th century BC, after the advent of the Deuteronomic movement.
3. The advent of monolatry
It is not clear when the Israelites began to make the momentous shift from polytheism to monolatry, but it may have been as early as the second millennium BC. The development may have had something to do with an event known as the "Exodus" and a national leader called Mosheh (Moses).
In the latter part of the 13th century BC, it seems that a large number of Levantines were living in Egypt, most of them working as slave labourers on building projects. Among these were many Israelites, and later generations of Israelites would claim that all of their ancestors had been held captive in Egypt. At some point, some or all of the captive Israelites seem to have been brought out of Egypt and into freedom under the leadership of Moses. Precisely when this event took place is unclear, but many scholars have placed it in the reign of the pharaoh Ramesses II (1279-1212 BC).
Can we go back even further? Jewish tradition attributes the ultimate origin of monotheism to the ancestral hero Abraham, a shadowy and semi-mythical figure who may have lived in the second millennium BC. In his book Judaism: The First Phase, Joseph Blenkinsopp has pointed out that there is no clear textual evidence of Abraham before the Babylonian Exile (c.587 BC), other than in the traditional formula "Abraham, Isaac, and Jacob". He also notes that the famous set of stories about Abraham in the Book of Genesis "mirrors to a remarkable degree the situation in Jewish communities in the Neo-Babylonian and early Persian period". The traditional notion that Abraham was the original Semitic monotheist remains both unproven and unprovable.
At some point after the Exodus, the doctrine seems to have developed of a supernatural contract or "covenant" between El-Yahweh and Israel. This notion would later assume great importance in Jewish and Christian thought.
4. Religion in the time of the kings (c.1000 BC - c.587 BC)
The shift to monolatry took centuries to accomplish, and did not initially result in a strict monotheism. During the high days of the Israelite monarchy, the religious spectrum ranged from something akin to developed Jewish monotheism to practices approximating to traditional Levantine polytheism. Patrick D. Miller has coined the terms "orthodox Yahwism", "heterodox Yahwism" and "syncretistic Yahwism" to describe these different tendencies (see my post here). For example, the worship of El's wife Asherah seems to have been quite common.
The Bible as we have it has been written and edited in a deliberate attempt to obscure the developments under discussion here, but there remains enough evidence in the text to give us some idea of what was going on. For example, the later tradition acknowledged, albeit with severe disapproval, that King Solomon had worshipped gods other than Yahweh:
For Solomon followed Astarte the goddess of the Sidonians, and Milcom the abominable god of the Ammonites. So Solomon did what was evil in the sight of Yahweh.... Then Solomon built a sanctuary for Chemosh the abominable god of Moab, and for Molech the abominable god of the Ammonites, on the mountain east of Jerusalem. (1 Kgs 11.5-7)There is some evidence that may date from this period that does seem to bear witness to something that looks like later monotheism:
…Who is god except Yahweh?
And who is a rock besides our god?... (Ps. 18.31)
Monotheistic-type beliefs were not necessarily typical, however. I have collected various examples of biblical texts bearing witness to polytheistic beliefs here.
As to the ritual and institutional side of the worship of Yahweh, various sanctuaries and cult-places existed, including Gilgal and Bethel in the north and Jerusalem in the south, where King Solomon built an enormous temple. One very important religious festival was the Pesach, or Passover. This probably developed from an ancient ritual in which a lamb was sacrificed during the lambing season in order to avert harm from the rest of the flock. It came, however, to be associated with the Exodus, which by now meant the delivery of all the Israelites from Egypt by Yahweh.
5. The prophetic reformation (c. 800 - 700 BC)
The "prophetic reformation" is the name given by the great Biblical scholar Julius Wellhausen to the religious movement which seems to have developed in the 8th century BC and found expression in the books of Amos, Hosea, Micah and Isaiah. It did not seek to change traditional Yahwism into something radically different: it rather re-emphasised and developed a series of themes already present in Israelite religion. It seems to have been spearheaded by a small group of relatively marginal activists, but its subsequent influence was immense.
The reforming prophets’ teachings are pervaded by their conviction that Yahweh is the sovereign deity of the universe. At times, they even seem to deny the existence of other deities, and deride their images and cultic apparatus as the work of human beings alone - "they bow down to the work of their hands", Isaiah complains of his countrymen, "to what their own fingers have made" (2.8). Interestingly, however, the prophets did not always condemn images of Yahweh (see Is. 19.19), and they were in principle happy to acknowledge the existence of other supernatural beings: they make particular reference to winged serpents called "seraphim".
The reformers believed strongly that their god was active in history: most importantly, Yahweh had chosen Israel as his people and led her out of slavery in Egypt. The relationship between the deity and his people was intimate and exclusive. Worshipping other gods constituted a betrayal of this relationship and is described using the vocabulary and metaphors of adultery and prostitution (Hosea is exemplary here). The reforming prophets were profoundly dismayed by their people’s failure to pay homage to Yahweh alone.
6. The advent of monotheism - the Deuteronomists and the Exile
The effects of the prophetic reformation on popular belief were probably quite limited: ordinary Israelites no doubt continued to tell stories about Yahweh’s battle with the sea-monster Rahab, to worship other deities and to venerate the spirits of their ancestors. Such traditional beliefs were still quite deeply ingrained in Israelite society: it is noteworthy, for example, that theophoric names referring to the polytheistic deity Ba'al are attested in significant numbers in inscriptions from the 8th century BC.
It seems plausible that the episode that sealed the transformation of traditional Yahwism into monotheistic Judaism was the shattering experience of the Babylonian Exile (c.587-538 BC). Prior to this, however, much headway in the direction of monotheism had been made by the Deuteronomic movement and its supporter King Josiah (reigned 641-609).
The Deuteronomic school was another religious reform movement. It originated in roughly the mid-to-late 600s and persisted well into the 500s. It is associated in particular with the discovery (or composition) of the original edition of the biblical book of Deuteronomy in 622 BC. For an example of its theological outlook, see this monotheistic-sounding statement from Deut. 4.35-39:
To you it was shown so that you would acknowledge that Yahweh is God; there is no other besides him.... He brought you out of Egypt with his own presence, by his great power, driving out before you nations greater and mightier than yourselves, to bring you in, giving you their land for a possession, as it is still today. So acknowledge today and take to heart that Yahweh is God in heaven above and on the earth beneath; there is no other.In tandem with the development towards monotheism was the increasingly dominant view that Solomon’s temple in Jerusalem was the only place where sacrifices could legitimately be offered to the God of Israel.
One interesting text found within the Deuteronomic corpus is the so-called Song of Moses (Deut. 32-33). It is not entirely clear exactly where or when this text originated, but it contains what sound like monotheistic polemics:
[The Israelites] made [God] jealous with strange gods and provoked him with abhorrent things.
They sacrificed to demons, not El, to deities they had never known,
to new ones recently arrived, whom your ancestors had not feared.
On the other hand, the Song does still seem to acknowledge the existence and legitimacy of other deities:
When the Most High apportioned the nations, when he divided humankind,
he fixed the boundaries of the peoples according to the sons of God [i.e. the gods].
Yahweh’s portion was his people [i.e. Israel], Jacob his allotted share.
In summary, it may be said that the Deuteronomic school prepared the way for monotheism rather than being thoroughly and radically monotheistic in itself.
Even the combined weight of the Deuteronomic movement and the upheavals and religious reforms associated with the Exile did not entirely succeed in bringing about the complete triumph of monotheism. Michael Heiser has been insistent that polytheistic beliefs persisted into the post-exilic period (see here). We can also see from the Book of Job, which was probably composed between the 6th and 4th centuries BC, how old ideas of a polytheistic heavenly court continued to persist:
Now the day came when the sons of God came to present themselves before Yahweh - and Satan also came among them. Yahweh said to Satan, ‘Where have you come from?’ And Satan answered Yahweh, ‘From roving about on the earth, and from walking up and down in it”…Further on the subject of Satan, there are no references to a supremely evil demon of that name in the earliest parts of the Bible. The original conception seems to have been that of "a satan" as a troublemaking messenger of Yahweh, and some texts represent "a satan" or "the satan" as a kind of bad or malicious angel. It was not until after the Exile that the figure of Satan began to emerge in its familiar form. This development seems to have been influenced by contact with the Persian religion of Zoroastrianism, which revolved around the figures of the supreme and benevolent god Ahura Mazda and his arch-adversary, the evil Ahriman.