Hosea 5:3

Yahweh knew Israel well; He had not been deceived and fallen into a trap, as the Israelites had. Ephraim had played the harlot against her husband, the Lord, and had defiled herself by doing so (cf. Leviticus 18:20; Leviticus 18:24; Numbers 5:20; Numbers 5:27-28). Ephraim was the largest tribe in Israel and so, frequently, was a synonym for the Northern Kingdom (e.g., Hosea 4:17). Hosea may have referred to it here because this tribe was foremost in idolatry. [Note: Wood, "Hosea," p. 190.] It was part of the priests’ responsibility to distinguish between clean and unclean (Leviticus 10:10), but they had not done their job, so Israel had defiled herself.