Truly in vain, etc. An obscure and (if corruption exists anywhere) corrupt passage, which, however, it is hopeless to attempt to emend, as the corruption consists partly in wrong letters, partly in omitted letters or words (or both); and, moreover, the text employed by the Septuagint appears to have presented the same difficulty.
The latter point is especially noteworthy. It is far from proving that the traditional text is correct; what it does suggest is that the writings of the prophets were at first written down in a very insecure manner.
The rendering of the Authorized Version is substantially that of Hitzig, who explains "the multitude of [the] mountains," as meaning "the multitude of gods worshipped on the mountains"—too forced an expression for so simple a context.
It seems most natural to suppose (with Ewald, Graf, and Keil), a contrast between the wild, noisy cultus of idolatrous religions, and the quiet spiritual worship inculcated by the prophets. Compare by way of illustration, the loud and ostentatious demonstrations of Baal's ritual in 1 Kings 18:1-46; with the sober, serious attitude of Elijah in the same chapter.
The word rendered in the Authorized Version "multitude" has a still more obvious and original meaning, viz. "tumult;" and probably the Targum is not far from the true sense in rendering, "In vain have we worshipped upon the hills and not for profit have we raised a tumult on the mountains."