John Calvin on Isaiah 38:6

6. And I will deliver thee. Those who think that Hezekiah was sick during the time of the siege found an argument on this, that otherwise this promise would appear to be superfluous. But there is little force in that reasoning; for the Assyrian might have recruited his forces, and mustered a fresh army, at a later period, for the purpose of again invading Judea and attacking Jerusalem. The very defeat of which we have now read might have been a provocation to his rage and cruelty, so that the Jews had good reason for being continually alarmed at any reports which they heard. “Quand on leur apportoit nouvelles de l’ ennemi.” “When news were brought to them about the enemy.” That promise, therefore, is far from being superfluous, because along with life it promises protection from the enemy, against whom he would not otherwise have been secured, and may be regarded as an enlargement and increase of that blessing which the Lord promised to Hezekiah; as in the former chapter he promised abundance of fruits to accompany the deliverance. (Isaiah 37:30.)