The monotheism that characterizes Judaism began in ancient Israel with the adoption of Yahweh as the single object of worship and the rejection of the gods of other tribes and nations without, initially, denying their existence.
Most mainstream Old Testament scholars believe that the religion of the early Israelites was neither monotheistic nor polytheistic but “monolatrous.” While the existence of other gods was not denied, Israel was to worship no god but Yahweh.
Maybe just read some encylopedias if you want to know more about the history of monotheism