Hello soulcalm17

[soulcalm17]

I'd be interested to hear of cases where it goes the other way -- monotheism to polytheism.

Sure, Belacqua. I present you some cases.

In Hindu scripture, actually God is depicting as uniquely one and unseen entity. Not many people know about it I guess. But it was mentioned in Upanishad. Hinduism start from about 3000 BCE (some says 8000 BCE and even timeless according to Hindus people) and as time continue, it goes to polytheistic style and had millions of God. Here is the verses:

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At least some of the copy/paste/spam deleted.  Stop it! 

About Arabian Peninsula that are pagans, actually that was a proof that their paganism deviated from monotheism. According to history, Arabs were descendant of Ishmael, the son of Abraham who was also practice monotheism. And as time goes by, they deviate to paganism.

Okay, thanks for the warning and open this thread again.

Now, I'm write my self. The verses are as follow:
1. He is one only without a second (Chandogya Upanishad 6:2:1)
2. Of Him there are neither parents nor lord (Svetasvatara Upanishad 6:9)
3. There is no likeliness of Him (Svetasvatara Upanishad 4:19)
4. His form is not to be seen; no ones sees Him with the eye (Svetasvatara Upanishad 4:20)

And even God prohibited to worship at demigods as written in Bhagavad Gita 7:20:

"Those whose intelligence has been stolen by material desires surrender unto demigods and follow the particular rules and regulations of worship according to the their own nature"