The earliest people in West Asia 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. The Phoenicians and Canaanites may have sacrificed their children to him. Their most important goddess was Astarte, another fertility figure. The Hittites arrived later, around 2500 BC. They 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. In exchange God 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. 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. Others became Buddhists, and the old Sumerian polytheism more or less died out.
Jesus brought the next wave of monotheism to West Asia six hundred years later with the development of Christianity. Christianity was the first religion to insist that not only should you worship only one god, but the other ones were not really even gods at all, but demons.
This has some similarity to the Zoroastrian idea that some minor gods are on the side of the Lie. It’s also like the Jewish idea that you should only worship your own one God. But Christianity takes both ideas to more of an extreme. Christianity gradually took over most of the west of West Asia, the part that was under Roman control, but the Sassanian part of West Asia remained Zoroastrian until about 700 AD.
The most recent religious movement of West Asia is Islam, which was taught by the Prophet Mohammed in the Arabian peninsula in the late 600s AD. Islam draws from earlier West Asian religions, but is even more fiercely monotheistic than Christianity is, with the mantra “there is no God but God.”
The establishment of the Islamic Empire in the late 600s and 700s AD spread Islam from Spain to China, nearly wiping out Zoroastrianism, and even today most of that area still follows Islam. The conquest of Turkey by the Ottomans in the 1200s AD spread Islam to Turkey, Greece, and the Balkans as well.
The Usborne Book of World Religions, by Susan Meredith (1996). Easy reading.
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.