John Calvin Commentary Isaiah 42:15

"I will lay waste mountains and hills, and dry up all their herbs; and I will make the rivers islands, and will dry up the pools." — Isaiah 42:15 (ASV)

I will reduce mountains and hills to a wilderness. The Prophet means that all the defenses and military forces on which the wicked pride themselves will not prevent God from setting His people free. It was necessary that this should be added to the previous statements, because when we see enemies who are extremely 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 show that no power or army whatever can resist the Lord when He wishes to deliver His people. In short, he shows that there will be such a revolution that those who were formerly most powerful will be crushed and will gain nothing by all their attempts against Him.

This appears to me to be the plain meaning of this passage, and there is no need to enter 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 need to pursue such refinements, because he simply declares that God is sufficiently powerful to fulfill His promises and deliver His Church, because He will easily surmount all the difficulties that present themselves to our eyes. This statement also corresponds to other predictions which we have previously 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.