John Calvin Commentary Isaiah 38:6

"And I will deliver thee and this city out of the hand of the king of Assyria; and I will defend this city." — Isaiah 38:6 (ASV)

And I will deliver you. Those who think that Hezekiah was sick during the time of the siege base an argument on this, contending that otherwise this promise would appear superfluous. But there is little force in that reasoning, for the Assyrian might have recruited his forces and mustered a new army later to invade Judea and attack Jerusalem again.

The very defeat we have just read about might have been a provocation to his rage and cruelty, so the Jews had good reason to be continually alarmed by any reports they heard. That promise, therefore, is far from superfluous. Indeed, along with life, it promises protection from the enemy, against whom he would otherwise not have been secure. It may also be regarded as an enlargement and increase of that blessing which the Lord promised to Hezekiah, just as in the previous chapter he promised an abundance of fruits to accompany the deliverance (Isaiah 37:30).