The Jews had polythiestic tendancies right up until the Return. Right from when they were a nation (coming up out of Egypt), the Jews were always surrounded by polytheistic peoples. It is unclear how much that influenced their own worship, but all through the time of the Kings they often worshipped other gods.
It was during the Exile that they saw, possibly for the first time, another monotheistic religion: Zoroastrianism. It is debated as to whether the exiled priesthood co-opted that, but from the time of Ezra forwards, Judaism was determinedly monotheistic.
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