The Hebrews were monotheists who believed in only one god, while the Sumerians and Egyptians believed in many gods (they were polytheists).
While the Jews believed in a god who was omnipotent (all-powerful), omnipresent (all-present), and omniscient (all-seeing), the Sumerians and Egyptians worshiped gods who behaved very much like powerful human beings. These gods didn't shy away from committing murders, engaging in incest, having affairs, and plotting to overthrow rivals. For example, the Egyptian god Seth murdered his brother, Osiris, because he was jealous of him. Osiris, of course, was married to Isis, his sister. Here's more about the story: The Story of Isis and Osiris.
As for Sumerians, the god Enki was forefront in their worship; with his Mesopotamian equivalent, Ea, Enki was the creator of the world. He was also, in turn, a virile god who had an immense sexual appetite, and he enjoyed sexual encounters with various goddesses. There are also Sumerian accounts of Enki and Inana (the goddess of sex and war) battling each other for the right to rule civilization. Here's more about Enki: Enki/Ea, god of wisdom and the creator of the world.
The Hebrews, for their part, saw their god as infallible and holy. He was viewed as a god they could trust in a time of trouble. The Hebrews believed that their god had a plan for their lives and that his laws were to be obeyed for their own good. The Hebrews viewed their god as the epitome of perfection, one who didn't participate in the sometimes violent and lascivious activities the Sumerian and Egyptian gods engaged in.
A major difference between the Sumerian and Egyptian people also lay in how they saw their gods. The Sumerians saw their gods as deities who had to be placated and catered to, while the Egyptians saw their gods as deities who were largely benign and well-disposed towards humans. For the Sumerians, the most important gods largely kept a distance between themselves and humans, so worshipers often relied on intermediary gods to intercede for them. Since Sumerian city-states often had patron gods and goddesses, the Sumerian people often spent inordinate amounts of time placating these gods with sacrifices and prayers.
On the other hand, the Hebrews approached their god without the benefit of intermediaries. Although they had a priestly caste, the Hebrews largely believed in a personal experience with their god.
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