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Mixed belief was initially the case, was it not?
The people who were left in the land of the northern tribes after many were carried away were already very far from the God who brought there forefathers out of Egypt. They worshipped from the time of their first king in Dan and Bethel rather than in Jerusalem. They had priests other than the sons of Aaron or of Levi serving for them. This very confused remnant was then mixed with the assorted peoples brought into the land by the king of Assyria:
"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.
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.
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.
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.
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.
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.
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 [they served God and mammon], after the manner of the nations whom they carried away from thence." II Kings 17:23-33