The account of that religion's origin can be seen in the following account (2 Kings 17:22-41)
For the children of Israel walked in all the sins of Jeroboam which he did; they departed not from them; until the LORD removed Israel out of his sight, as he had said by all his servants the prophets. So was Israel carried away out of their own land to Assyria unto this day.The "sins of Jeroboam" refers to a collection of sins of which the principle examples were the golden calves and the unauthorized priesthood. From the time of Jeroboam, until the destruction of Israel with the permanent exile of the ten tribes, the Israelites (as a nation) never gave up this ungodly worship of the Lord.
And the king of Assyria brought men from Babylon, and from Cuthah, and from Ava, and from Hamath, and from Sepharvaim, and placed them in the cities of Samaria instead of the children of Israel: and they possessed Samaria, and dwelt in the cities thereof.The Assyrians were, in some ways, smart. By reshuffling and intermixing the people, they avoided the ancient loyalties and helped to reinforce an Assyrian empire identity. The result, however, was that there were essentially no "native" Israelites in Israel.
And so it was at the beginning of their dwelling there, that they feared not the LORD: therefore the LORD sent lions among them, which slew some of them.The people of the land recognized that they were receiving divine judgment in the form of these lions. Being from other places that each had its own "god," they assumed that there must be some local deity in Israel that they needed to appease. However, no one knew how to appease the local deity. So, they begged for the king of Assyria's help.
Wherefore they spake to the king of Assyria, saying, "The nations which thou hast removed, and placed in the cities of Samaria, know not the manner of the God of the land: therefore he hath sent lions among them, and, behold, they slay them, because they know not the manner of the God of the land."
Then the king of Assyria commanded, saying, "Carry thither one of the priests whom ye brought from thence; and let them go and dwell there, and let him teach them the manner of the God of the land."
The king of Assyria had a solution. Send one priest back to teach them how to worship this local deity.
Then one of the priests whom they had carried away from Samaria came and dwelt in Bethel, and taught them how they should fear the LORD.So, from this unauthorized Israelite priesthood, a single priest returned to teach them how to worship the Lord - not how to worship Baal, but the Lord. This suggests that the worship of the Jeroboamic religion was a faulty worship of the Lord.
And unsurprisingly, this one priest taught them how to worship the Lord, but did not teach them to worship the Lord alone. While God seems to have accepted this fundamentally unacceptable co-worship in terms of stopping the lion attacks, the text makes clear that this joint worship of God and other gods was not acceptable:
Howbeit every nation made gods of their own, and put them in the houses of the high places which the Samaritans had made, every nation in their cities wherein they dwelt. And the men of Babylon made Succothbenoth, and the men of Cuth made Nergal, and the men of Hamath made Ashima, and the Avites made Nibhaz and Tartak, and the Sepharvites burnt their children in fire to Adrammelech and Anammelech, the gods of Sepharvaim.Did God save these pagans who worshiped the Lord? God does not tell us that explicitly. It seems that the people did not continue to cry out to the king of Assyria for something more, nevertheless it is clear from the text that we should not view what they did as good enough.
So they feared the LORD, and made unto themselves of the lowest of them priests of the high places, which sacrificed for them in the houses of the high places.
They feared the LORD, and served their own gods, after the manner of the nations whom they carried away from thence. Unto this day they do after the former manners: they fear not the LORD, neither do they after their statutes, or after their ordinances, or after the law and commandment which the LORD commanded the children of Jacob, whom he named Israel; with whom the LORD had made a covenant, and charged them, saying,Ye shall not fear other gods, nor bow yourselves to them, nor serve them, nor sacrifice to them: but the LORD, who brought you up out of the land of Egypt with great power and a stretched out arm, him shall ye fear, and him shall ye worship, and to him shall ye do sacrifice. And the statutes, and the ordinances, and the law, and the commandment, which he wrote for you, ye shall observe to do for evermore; and ye shall not fear other gods. And the covenant that I have made with you ye shall not forget; neither shall ye fear other gods. But the LORD your God ye shall fear; and he shall deliver you out of the hand of all your enemies.Howbeit they did not hearken, but they did after their former manner. So these nations feared the LORD, and served their graven images, both their children, and their children's children: as did their fathers, so do they unto this day.
I should also point out that there is a slight apparent contradiction you may have noticed "they feared the Lord" and "they fear not the Lord." The resolution of this apparent contradiction is seen in the fact that while they do outwardly give worship to the Lord, nevertheless they do not do so according to the way that the Lord commanded. This single priest of Israel was not one of God's appointed priests. He did not properly instruct the people of the land, nor - if he did - did they properly follow his instruction.
I suppose we ourselves can take a warning from this. The warning would be to be mindful that we are not content simply to have some general worship for God, but also to follow his commandments. After all, it is one thing to be afraid of God's lions, but it is another thing to love the law of God.