It’s implicit in the beliefs in the three related religions of Islam, Judaism and Christianity that the one true God was known and worshipped from the beginning of time. Or at least from the time of Abraham, which is almost the same. (Bishop Ussher of Armagh calculated that Abraham was born 1943 years after the Creation.)
Abraham, if he existed, would have lived some time in the earlier half of the second millennium BC, say before 1500 BC. But the difficulty is that the stories about him (and the rest of the Bible) weren’t written down until a full thousand years later, when monotheistic Judaism was fully established. That makes it hard to use the Bible to learn any definite facts about the beliefs and practices of that earlier time.
There are a few mentions in the Bible that could be remaining early references to multiple Gods. A couple of times, the Jewish God is referred to as God of Gods, or the Greatest of the Gods. The plural word for “Gods” pops up too, in contexts hard to reconcile with monotheism. And Elohim, a title for the one God, is also a plural, although sometimes used with a singular verb.
To the sceptic, it’s implausible that monotheism existed in the second millennium BC: all the archaeological and historical evidence shows multiple Gods. In the general area of what is now Palestine and Israel, a large number of Gods were worshipped, but usually it was acknowledged that there was a supreme King and Queen of Heaven, El and his wife Asherah, to whom all other Gods were subordinate.
“El” came to be the noun in Hebrew simply meaning “a God”. It could be used to refer to Jehovah, the tribal deity of the Hebrews, but equally to Gods of neighbouring tribes, Ba’al, Moloch and a host of others. The original El was sometimes called Elyon, God on High, to distinguish him, but confusingly, this title came to be adopted for Jehovah too.
But it doesn’t seem to be the case that the developing Jewish religion directly took El and made him their one God. It was much more complicated. In fact, it was more the other way round — Jehovah absorbed the attributes of the better-known El, including his having a wife Asherah.
Even according to the Bible, Asherah’s statue was worshipped in Jehovah’s temple in Jerusalem right up until the Babylonian conquest in 587 BC, although a couple of the kings in the preceding centuries, Josiah and Hezekiah, had conducted purges and tried to enforce Jehovah-only worship. It never stuck. The second book of Kings is full of fascinating detail of the religious practices that went on, including sacred horses of the Sun and temple prostitutes in Jehovah’s house.
In fact, it was only in the aftermath of that defeat by the Babylonians and the exile of the king and nobility that a consistent Jewish theology was developed, and the Bible written up and edited to conform (mostly). When the kingdom was re-established and Jehovah’s temple in Jerusalem rebuilt in 515 BC, worship was completely monotheist, although, ironically, there were many competing sects within Judaism: Pharisees, Sadducees, Essenes, and eventually, Christians.