John Calvin Commentary Isaiah 48:21

"And they thirsted not when he led them through the deserts; he caused the waters to flow out of the rock for them; he clave the rock also, and the waters gushed out." — Isaiah 48:21 (ASV)

Therefore they thirsted not. Because the Jews did not see the way opened for their return, and because great and dangerous wildernesses intervened, the Prophet asserts God's power and brings forward examples of it, so that they would not be terrified by any difficulty.

He therefore urges them to consider whether or not God had sufficient power to rescue their fathers from slavery in Egypt and to lead them through desolate wildernesses, where He supplied them with food, water, and everything necessary for them (Exodus 16 and 17; Numbers 20).

Here the Jews, according to their custom, devise absurd fables and invent miracles that were never performed. They do this not through ignorance, but through presumption, by which anything plausible, even if there is no basis whatever for it, easily gains their support.

The Prophet's purpose was to call to their remembrance the former departure from Egypt and the miracles the Lord performed at that time—a practice we have already noted as customary for the Prophets when they wish to extol the works of God in lofty terms. Thus David, when celebrating the victories he had obtained, says that

the mountains trembled and flowed down, that the air was cleft asunder, and that the Lord was seen from heaven,
(Psalms 18:7).

Even though nothing of this kind ever happened to him, he imitates the description of the deliverance from Egypt to show that God, who was its author, had also been his supporter and leader in conquering his enemies. He does this to show that God's power ought to be acknowledged no less in his victory than in those signs and wonders.

Similarly, the Prophet wants the people now to contemplate those miracles to correct their unbelief and so that they might not be tempted by any distrust.

God's holy servants were always accustomed to cast their eyes on that deliverance so that, by remembering so great a benefit, they might strengthen the hearts of all in hope and confidence.

As we have said before, it was the duty of believers in every age to expect the fruit of this redemption, so that the Lord, by uninterrupted progress, might be the guardian of a redeemed people.

Thus Isaiah means that the Lord will easily overcome every obstacle, open up a way that is closed, and supply them abundantly with water, so that they will not die of thirst. This is just as He formerly brought water out of a rock by an extraordinary miracle when the people thought their condition was hopeless.

Consequently, there is no reason for them to despair of their return, if they are willing to contemplate and sincerely believe in that power of God which they have already experienced.