Monotheism and Polytheism
The earliest people that we know of were all polytheistic: they all worshipped many gods. From 3000 BC to 539 BC, the Sumerians, the Akkadians, the Assyrians and the Babylonians all worshipped pretty much the same set of gods, despite their cultural differences. The most important of these gods was Ea. Ishtar was the most important goddess. Like the Greek Aphrodite and Demeter, or the Roman Venus and Ceres, or the German Freya, Ishtar was a fertility goddess.
The Phoenicians and Canaanites, further west along the Mediterranean coast, were also polytheistic, but they had different gods. Their most important god was Baal, and some reports say that the Phoenicians and Canaanites sacrificed their children to him. Their most important goddess was Astarte, another fertility figure. The Hittites arrived later, around 2500 BC, and had different gods because they were Indo-Europeans, but they were polytheistic too.
The first signs of monotheism in West Asia come from the Bible, where by around 1000 BC the Jews seem to have already thought that they should worship only their own one God. They clearly believed that there were many gods, but they should only worship theirs, and in exchange he would take care of them against all the other gods. They may have gotten this idea from the Egyptians.
The next move toward monotheism comes from Zoroastrianism, also around 1000 BC. In Zoroastrianism the main god was Ahura Mazda, and his twin sons represented the Truth and the Lie; all the minor gods were on either the side of Truth or the side of the Lie. The most important of these minor gods was Mithra, who was the god of treaties and contracts, and of civilization
When the Persian king Cyrus converted to Zoroastrianism and then conquered a huge empire, many of his subjects also became Zoroastrians, and the old Sumerian polytheism more or less died out.
To read more about polytheism and monotheism, check out these books from Amazon.com or from your library:
The Usborne Book of World Religions, by Susan Meredith (1996). For kids.
Religion in Ancient Mesopotamia, by Jean Bottero (2001).
God Against the Gods: The History of the War Between Monotheism and Polytheism, by Jonathan Kirsch (2004). From Akhenaten in Egypt, through Judaism and the rise of Christianity. Lively, popular writing.
A World Full of Gods: The Strange Triumph of Christianity, by Keith Hopkins (2001). Entertaining account of what it was really like at non-Christian and early Christian religious events. Not for young kids.