So these nations feared YHWH and also served their carved images. Their children did likewise, and their children's children—as their fathers did, so they do to this day.
2 Kings 17:41

They believed in God, but they didn't believe God. Partly because they had no one to teach them who God is and what he desires. It's not enough to fear YHWH. You have to obey his commandments, which means you have to seek them out and verify what you have been told.