Albert Barnes Commentary 2 Kings 18:33

"Hath any of the gods of the nations ever delivered his land out of the hand of the king of Assyria?" — 2 Kings 18:33 (ASV)

The boast is natural. The Assyrians had an uninterrupted career of success and might well believe that their gods were more powerful than those of the nations with whom they had waged war. It is not surprising that they did not understand that their successes until now had been allowed by the very God, Yahweh, against whom they were now boasting.