John Calvin on Isaiah 29:6

6. From Jehovah of hosts shalt thou be visited. He next assigns the reason why all this multitude of garrisons shall be “like chaff;” and he expresses this by an opposite metaphor, for with those soldiers he contrasts the anger and “visitation of the Lord.” What is “chaff” to the flame of “a devouring fire?” What is “dust” to the force and violence of a “whirlwind?” He shews that the vengeance of God will be such as all their preparations shall be unable to resist. This meaning, in my opinion, makes the passage to flow easily, and the clauses will not be so well adjusted, if we follow a different interpretation.

Hence we learn that those who assail us can do no more than what the Lord permits them to do. If therefore the Lord determine to save us, the enemies will accomplish nothing, though they raise up the whole world against us. On the other hand, if he determine to chastise us, we shall not be able to ward off his wrath by any force or bulwarks, which shall quickly be thrown down as by a “whirlwind,” and shall even be consumed as by “a flame.”