Albert Barnes Commentary 2 Kings 18:29-30

"Thus saith the king, Let not Hezekiah deceive you; for he will not be able to deliver you out of his hand: neither let Hezekiah make you trust in Jehovah, saying, Jehovah will surely deliver us, and this city shall not be given into the hand of the king of Assyria." — 2 Kings 18:29-30 (ASV)

There were two grounds, and only two, on which Hezekiah could rest his refusal to surrender:

1. Ability to resist by his own natural military strength and that of his allies.
2. Expectation of supernatural assistance from Yahweh, based on the words of the prophet Isaiah (Isaiah 30:31; Isaiah 31:4–9).

The Rab-shakeh argues that both grounds for confidence are equally fallacious.