The Biblical authors, if we can trust them, are always railing against fellow Hebrews for partaking in worship of various deities besides Jehovah/El—the goddess Asherah and god Baal are two mentioned by name. (Although it must be noted that the Hebrews may have been henotheistic or polytheistic at some points in history. Monotheism may have been a minority opinion.)
5th century Britain is another instance, with the arrival of the Anglo-Saxons.
What else?
I find this subject endlessly fascinating,
The OT authors are attempting to reform Judaism into monolatry or monotheism, a Yahweh-only cult if you will, depending on who is writing and when. When Yahweh says "You shall have no other gods before me," it doesn't mean "You cannot worship other gods," it simply means "Worship me above all others." It's an admittance that other gods exist, because you don't have to ban people from doing something that they can't do (or you could, if you want to appear omnipotent and angry, c.f. "your days are numbered to 70 years"), e.g. passing a law that it is illegal to use Lightsabers or Blasters. The Asherah and Baal worship usually went hand-in-hand with Yahweh worship. Generally speaking, the cult was Yahweh as chief deity and war god, Asherah as Queen of Heaven, and Baal as their storm god son.
An excerpt from a lecture I gave a month or so ago that explains the above a little more clearly:
In Jeremiah 44:19, the Asherah worship has been ended at this point by Josiah, and the women are complaining. Do you remember their complaint? Not that they can no longer worship Asherah, although they are upset about this, but that they are being specifically targeted as the worshippers of Asherah. They complain that they were not alone in offering sacrifice to Asherah, that their husbands offered sacrifice with them. Here’s how the NIV reads: “…Queen of Heaven…” Queen of Heaven, that’s an interesting title. What it amounts to is that these folks worshiping at altars across Israel and Judah were worshipping Yahweh and his wife, Asherah. Yahweh was the warrior god, Asherah was the fertility goddess, and in some places, not all but some, they also worshiped the son of Yahweh and Asherah whose name was Baal.
[L]ook at [the] verses about the kings and the high places. Notice a trend? They did right [in the eyes of the Lord] but they didn’t remove the high places. Even Joash, who was advised by a priest so important that that priest’s name is recorded in the annals, even Joash did not take them down. We must ask ourselves, why? You might hear someone say that it was politically expedient for them not to, but I don’t think that’s the case. I think it’s because Exodus 20:24 gives implicit permission to worship Yahweh in many places. Why take down the high places if they’re acceptable? And so long as they’re worshiping Yahweh first and foremost, they’re okay.
So it wasn't that the Hebrews were turning away from Yahweh-only worship, it's that they had always been doing so. If you're familiar with the "United States Forefathers were Christians who founded a Christian nation" revisionist history, you can imagine that the priests and scribal schools of the day were doing something similar. Reframing the history to make it appear different than it actually was.
For further reading on the history of Israel and the JEDP(R), try some of these links (highly recommended links in bold):
Scholarly consensus (or majority belief) on the Bible authenticity?
How do scholars determine the age and origins of Old Testament stories?
Do we know who the 13 tribes of Israel were? - synopsis of historical data and a conservatively plausible narrative based on it
How did Judaism form? - a big discussion on the polytheistic elements of the Torah/ OT
You'll want to look into the Deuteronomistic history as well. The starting point for almost everyone is Canaanite Myth and Hebrew Epic by Frank Moore Cross.
The Imam-shahi Ismailis would be of interest to you. It's a faith that's a mix of Nizari Ismaili Islam and Hinduism whose adherents live in Gujarat — and who claim a variety of identities. Traditionally they practiced taqiyya by telling the Sunni authorities they were Hindus. Today about 85% put Hindu on the census form and 15% put Muslim.
Source: Moir, Zawahir and Khan, Dominique-Sila(2010) 'New Light on the Satpanthi Imamshahis of Pirana', South Asia: Journal of South Asian Studies, 33: 2, 210 — 234
I would recommend reading chapter 14 - "The People's Religion" from Rodney Stark's The Triumph of Christianity. I don't have my copy with me at the moment, but portions of the text are available online here: http://www.harpercollins.com/web-sampler/9780062007681 (Expand to fullscreen for easy navigation)
This chapter provides a concise, popular-level view of the trends in European popular religion from the fall of Rome to the Protestant Reformation. He discusses how the vast majority of the population lacked identifiable Christian commitment and clung to pagan rituals and syncretized the faith preached by the Church with the ancient traditions of their community.