Jerome : (Verse 33, 34.) Therefore, thus says the Lord about the king of Assyria: He shall not enter this city, nor shoot an arrow there, nor come before it with a shield, nor build siege works against it. By the way that he came, by the same he shall return, and he shall not enter this city, says the Lord. I will defend this city and save it for my own sake and for the sake of my servant David. It turns back to its purpose and dispels present fear after the hope of future things. For there was not so much joy upon the things which she had promised would come after a long time, as there was care for the impending. But what she says about Assyria returning and not lifting up a shield against Jerusalem, not shooting arrows, nor that the city should be fortified with walls and a rampart, and that the enemy would return by the way he had come, and that the city should be freed from the present siege, and finally she brings forth: for my sake and for the sake of my servant David, this signifies that they are preserved not by their own merit, but by the clemency of God, indeed by the memory of their father David. In which both his own negligence and that person's trustworthiness and justice are admonished, because God loves justice to such an extent that he protects even the descendants of holy men, not by their own merit, but by the virtue of their ancestors. — Commentary on Isaiah