In the Old Testament of the Bible, Yahweh is the one God of the Israelites. From the time (around 1900 BCE) Abraham left Ur for Canaan, the nomadic Hebrew tribes are described as a monotheistic people. And when Moses made a covenant with Yahweh at Mt. Sinai after the Exodus from Egypt in the 13th century BCE, the tribes became united under their one God and the laws he gave to them. Historical evidence, however, suggests the Israelites didn’t completely adopt their monotheistic religion until 538-515 BCE - when they returned to Jerusalem from their exile in Babylonia.
Ugaritic texts, discovered in the ruins of the ancient city of Ugarit in 1928, have shed new light on the origins and development of the early Jewish religion. The chief god was El, and his wife was Asherah. They had seventy divine children, characterized as the stars of El. Among them was Baal, Astarte, Anat, the sun-goddess Shapshu, the moon god Yerak, and Yahweh. The servants of the divine household of the gods were messenger-gods. These appear to be what the Old Testament later refer to as angels.
In this early stage of the Israelite religion El divided the nations of the Earth among his divine family and they became the patron deities of the seventy nations. Yahweh was given Israel. After some time the god El became identified with Yahweh, and the result was that Yahweh-El was the husband of Asherah. In this form Yahweh was the Divine King ruling over all the other gods. Between the 8th and 6th centuries BCE the other gods were relegated as mere expressions of the power of Yahweh, and his divine messengers became minor divine beings subservient to him.
Why did these changes take place? It has been suggested that the Israelites original view of the world was that each patron god was as powerful as its nation. This sat well when Israel was on a par with its neighbors. The rise of the Assyrian and Babylonian empires, however, implied that the Hebrew god was not as powerful as had been traditionally thought. This forced the Israelites to alter the religious way they looked at the world, and the new thinking separated heavenly power from earthly kingdoms. Although Israel was weak, its god was not. Yahweh came to be divine power, and the Mesopotamian gods were nothing. Israel had been conquered not by Assyrian and Babylonian power or the power of their divine patrons, but because Yahweh, the one true God, was guiding all the events of the one nation he had chosen. They were being punished and purified.