John Calvin Commentary Isaiah 30:31

"For through the voice of Jehovah shall the Assyrian be dismayed; with his rod will he smite [him]." — Isaiah 30:31 (ASV)

Surely by the voice of Jehovah. He added this for two reasons. First, to show why the Assyrian must be bruised; for, since he was cruel and savage to others, it is proper that “the same measure which he meted should be measured to him again” (Matthew 7:2).

This is the ordinary judgment of God against tyrants, as the Prophet says in a subsequent passage of this book, “Woe to thee that spoilest, for thou shalt be spoiled” (Isaiah 33:1).

The second reason is because the power of the Assyrian king appeared to be so great that he could not fall. Although, therefore, he was fortified on every side, not only to defend himself but also to attack others, yet the Prophet says that “by the voice of God” alone he will be bruised.

Therefore, we learn how groundless is the confidence of wicked men, who rely on their garrisons and arms and presumptuously despise God, as if they were not liable to his judgment. But in order to destroy them, the Lord will have no need of any other arms than his own “voice.” For by the slightest expression of his will, he will lay them low.

Nor can it be doubted that the Prophet intends to withdraw the minds of believers from earthly means, so that they may not inquire how it will be done, but may be satisfied with the bare promise of God, who is fully able to execute his word as soon as it has gone forth from him.