John Calvin Commentary Isaiah 21:9

"and, behold, here cometh a troop of men, horsemen in pairs. And he answered and said, Fallen, fallen is Babylon; and all the graven images of her gods are broken unto the ground." — Isaiah 21:9 (ASV)

Babylon is fallen, is fallen. This shows plainly that it is not king Belshazzar’s watchman who is introduced, for this speech would be unsuitable for such a character. The Prophet therefore makes known, by the command of God, what would happen. Now, this may refer either to God or to Darius, as well as to the watchman; and it makes little difference to the meaning, for Darius, being God’s servant in this matter, is not inappropriately represented as the herald of that judgment. There would be greater probability in referring it to God himself, for Darius had no such thoughts when he overthrew the idols of the Babylonians. But the speech agrees better with the role of a guardian, as if an angel added an interpretation to the prophecy.

And all the graven images of her gods. There is here an implied contrast between the living God and dead idols. This mode of expression, too, deserves notice, when he calls them “images of gods.” For the Babylonians knew, as all idolaters loudly proclaim, that their images are not gods. Yet they ascribed to them divine power, and when this is done, the truth of God is changed into a lie (Romans 1:25), and not only that, but God himself is denied. But on this subject we will speak more extensively later. Here we see that by her destruction Babylon was punished for idolatry, for he assigns the reason why Babylon was destroyed. It was because the Lord could not endure that she should glory in her “graven images.”