John Calvin on Isaiah 42:15

15. I will reduce mountains and hills to a wilderness. The Prophet means that all the defences and military forces on which the wicked plume themselves shall not prevent God from setting his people at liberty. It was necessary that this should be added to the former statements; for when we see enemies exceedingly powerful, and almost invincible, we tremble, and do not look for God’s assistance, which would be necessary to keep our faith strong. On this point, therefore, the Prophet dwells, in order to shew that no power or army whatsoever can resist the Lord when he wishes to deliver his people. In short, he shews that there shall be such a revolution, that they who formerly were most powerful shall be crushed, and shall gain nothing by all their attempts against him.

Such appears to me to be the plain meaning of this passage, and there is no necessity for entering into ingenious speculations, as some have done, who, in an allegorical interpretation of these words, pronounce that by “mountains and hills” are meant cities, and by herbage the men who inhabit them. But there is no necessity for pursuing such refinements; for he simply declares that God is sufficiently powerful to fulfill his promises and deliver his Church, because he will easily surmount all the difficulties which present themselves to our eyes. This statement corresponds also to other predictions which we have formerly seen, in which the Prophet taught that as soon as God has determined to assist his people, his power is not limited to natural means, but miraculously breaks through every obstruction that appears to hinder his passage.