in vain, &c. = as certainly as the hills [have proved] false, and the noisy throng on the mountains fan empty sound], so truly is the salvation of Israel with our God. The "hills" and "mountains" are put by the Figure of speech Metonymy (of Subject), for the idolatry practiced on them. Compare Eze_18:6, Eze_18:11, Eze_18:15.