This Man would like some words with you.
Also, look up ancient (as "in the Time of Moses") Judaism, Roman Christianity, the first Muslims, and Zoroastrianism.
Monotheism has existed for most of its history alongside polytheism. Most of the time, they're the minority, and when they are the majority, it's with a vengeance.
I've got a D&D setting where the main characters are mostly monotheistic, but debate on what is the "true monotheism," because although there is divine magic, divine magic, it turns out, isn't much of a proof of a God in a world where 700 other kinds of magic exist as well.
"I see you cast spells by praying. I know a guy who casts spells by reading passages of famous plays. A buddy of mine casts spells by eating junk he finds laying around his house. My grandfather cast spells by getting drunk. I hear the Gnostics cast spells by scarring their flesh. You do it by saying the word "God" over and over again? And that's supposed to prove you know more about the world than my wood-eating buddy or my alcoholic grandfather? Get in line, holy man."
The setting also has angels and devils who run around, and you can go visit Hell if you'd like, and there is a firmament with heavens above....of course, most of what's up there and down there isn't exactly explained well to the mere mortals who scrounge around in the material world. Why doesn't God define himself better? Why do some angels not believe in Him? Why is there a giant fanged monster frozen in ice at the center of the world? All part of the Great Mystery. Maybe those pagans have something after all...