Did the Kohen serve Hashem, or idolatry?
Malbim: He served idolatry. However, Yisrael who served idolatry believed that Hashem is the Supreme G-d, just they considered idols and the calves to be intermediaries. Shituf (serving something else with Hashem) is forbidden to Yisrael; we may serve "Bilti la'Shem Levado" (Shemos 22:19), for we are not under any Mazal or star. For the Kusim, this sufficed for them to be called Yir'ei Hashem - "mi'Mizrach Shemesh v'Ad Mevo'o Gadol Shmi ba'Goyim" (Mal'achi 1:11).
It says that the Kohen taught them how to fear Hashem. Why does verse 29 say "Goy Goy Elohav"?
Radak: Even though each nation will serve its god, their hearts will be to Hashem. Also, they should avoid abomination such as homosexuality, bestiality and incest.
