John Calvin Commentary Isaiah 37:21

"Then Isaiah the son of Amoz sent unto Hezekiah, saying, Thus saith Jehovah, the God of Israel, Whereas thou hast prayed to me against Sennacherib king of Assyria," — Isaiah 37:21 (ASV)

Then Isaiah sent to Hezekiah. This shows the result of the prayer; for, as soon as matters reached an extreme, God suddenly holds out his hand to assist the pious king through the Prophet Isaiah (2 Kings 19:20). Not that he immediately stretches out his arm to drive away the enemies, but he promises deliverance by the mouth of the Prophet, and thus even now calls his servant's faith into action. Isaiah undoubtedly could not by himself offer any assistance, and therefore it would have been foolish for him to promise this or that if Hezekiah had not been convinced that God had sent him. Thus, until God gave a manifestation of his power, he rested satisfied with this consolation.

Thus says Jehovah the God of Israel. Here we are taught that we should always inquire of God if we wish to obtain any relief from our anxieties and distresses; for if we reject the doctrine which he communicates to us through faithful teachers, we are utterly unworthy of receiving any consolation.

Fed and nourished by it, we should make continual progress and seek new confirmations from it. This ensures that new remedies may be constantly found for new distresses, and that we may never lack consolation even in the midst of the sharpest afflictions. Indeed, even those whose resources and means of defense are most extensive cannot be too abundantly supplied with this doctrine.

In Hezekiah, a striking instance of faith and steadfastness is displayed here. The Lord does not merely comfort him once or only by a single prophecy; he also confirms him by many prophecies. This is so that we, who are far removed from such steadfastness, may know that we need many and various aids to give uninterrupted support to our faith.

Since you have prayed to me. In the sacred history (2 Kings 19:20), the statement I have heard, is used. Consequently, in that passage, אשר (asher) is a relative pronoun; but here, as in many other passages, it is introduced either for explanation or for assigning a reason. To supply the statement I have heard, as some commentators do, is harsh and unnatural. The sentence flows on in unbroken connection when God declares that he grants it as an answer to Hezekiah's prayers, and that he will frustrate all the efforts of the tyrant and restrain his violence and rage, as if he had said that God’s answer corresponded to the pious king's prayers.

Indeed, whoever addresses prayers to him will eventually experience how ready he is to answer. However, very frequently he is silent and offers no consolation from his word, because in the midst of our distresses we are silent.

Concerning Sennacherib king of Assyria. The prediction amounts to this: there is no reason why Hezekiah, conscious of his destitution and weakness, should faint or despair when he sees the insolence of this haughty tyrant, because God will intervene between them. Since Sennacherib offered those insults to the afflicted Jews, God declares that he takes this cause into his own hands, because the affront was directed against himself.

By these words, he shows that he will take vengeance when his grace is despised by unbelievers. He also advises believers not to be greatly distressed about being despised by the world, provided that their weakness finds assistance readily prepared in heaven.