Ephraim is joined to idols: let him alone.
Ephraim - the ten tribes. Judah was at this time not so given to idolatry as afterward.
Is joined to idols - closely and voluntarily; identifying themselves with them as a whoremonger becomes one flesh with the harlot (Numbers 25:3, "Israel joined himself unto Baal-peor;" 1 Corinthians 6:16-17, "What know ye not that he which is joined to an harlot is one body? for two, saith He, shall be one flesh. But he that is joined unto the Lord is one spirit").
Idols - the Hebrew х `ªtsabiym (H6091)] means also sorrows, pains, implying the pain which idolatry brings on its votaries: with pains the idols are made (Isaiah 44:12-13), and pain is the portion of their worshippers.
Let him alone - leave him to himself. Literally, give him rest-i.e, from all further expostulations, which he will not hear (Pusey). Let him reap the fruits of his own perverse choice: his case is desperate: have you nothing to say to him (cf. Jeremiah 7:16, "Pray not thou for this people, neither lift up cry nor prayer for them, neither make intercession to me: for I will not hear thee"). Here Hosea 4:15 shows the address is to Judah, to avoid the contagion of Israel's bad example. He is bent on his own ruin; leave him to his fate, lest, instead of saving him, thou fall thyself (Isaiah 48:20; Jeremiah 50:8; Jeremiah 51:6; Jeremiah 51:45; 2 Corinthians 6:17).