Should he merely inform them of their abominations?
Rashi: Yes.
Radak citing Targum Yonasan: He should rebuke the residents and show to them their abominations.
Radak (3): He should rebuke them via a Mashal.
Malbim: Hashem's relation to Yisrael is compared to one who found an abandoned girl. She was cast in a desolate Midbar. A Nediv (generous person) passed by and had mercy on her, and decided to do good to her. He raised her, married her and bestowed all good on her. She forgot all that he did for her, and was Mezanah under he must to anger him ? "Yimtza'ehu b'Eretz Midbar? Yitzerenhu k'Ishuon Eino; ? Yarkivehu Al Bamasi Aretz; ?va'Yishman Yeshurun va'Yiv'at?" (Devarim 32:10, 13, 15).
To whom should he say so?
Radak: Even though he was in Bavel, he must inform Bnei Yerushalayim via a Sefer ? perhaps they will heed and repent.
Radak citing Targum Yonasan: Say [to Bnei ha'Golah] 'so Hashem Elokim said about those who dwell in Yerushalayim?'
