Even while these people were worshiping the Lord, they were serving their idols

 2 Kings 17:24-41

The king of Assyria brought people from Babylon, Kuthah, Avva, Hamath and Sepharvaim and settled them in the towns of Samaria to replace the Israelites. They took over Samaria and lived in its towns. When they first lived there, they did not worship the Lord; so he sent lions among them and they killed some of the people.

Then the king of Assyria gave this order: “Have one of the priests you took captive from Samaria go back to live there and teach the people what the god of the land requires.” So one of the priests who had been exiled from Samaria came to live in Bethel and taught them how to worship the Lord.

Nevertheless, each national group made its own gods in the several towns where they settled, and set them up in the shrines the people of Samaria had made at the high places.

They worshiped the Lord, but they also appointed all sorts of their own people to officiate for them as priests in the shrines at the high places. They worshiped the Lord, but they also served their own gods in accordance with the customs of the nations from which they had been brought.

To this day they persist in their former practices. They neither worship the Lord nor adhere to the decrees and regulations, the laws and commands that the Lord gave the descendants of Jacob, whom he named Israel. 

They would not listen, however, but persisted in their former practices. Even while these people were worshiping the Lord, they were serving their idols. To this day their children and grandchildren continue to do as their ancestors did.

Over the course of some years, Assyria repopulated the land with other conquered peoples, each with its own religion. But troubles are attributed to their failure to placate the local god, and an Israelite priest is sent back as a missionary. From this strange hotchpotch of religions a purer form of worship emerged amongst their descendants, the Samaritans who nonetheless, right through to New Testament times.

Why?

Even a priest was sent to Israel as a missionary, but people who were living in Samaria worshiped the Lord and other gods. This is a general popular regulation model, people choose anyone they like to worship. They forgot what the Lord has told them.

Israelites were exiled to other countries and other people immigrated to Samaria. Until the New Testament people were living there still doing what they like to do.

How?

Our God is different from others, full of love and righteousness, My Lord is the fountain of blessing.