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

(1) ability to resist by his own natural military strength and that of his allies; and

(2) expectation based upon the language of Isaiah Isa 30:31; Isaiah 31:4-9, of supernatural assistance from Yahweh.

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

Thus saith the king, Let not Hezekiah deceive you: for he shall not be able to deliver you out of his hand:

18:17-37 Rabshakeh tries to convince the Jews, that it was to no purpose for them to stand it out. What confidence is this wherein thou trustest? It were well if sinners would submit to the force of this argument, in seeking peace with God. It is, therefore, our wisdom to yield to him, because it is in vain to contend with him: what confidence is that which those trust in who stand out against him? A great deal of art there is in this speech of Rabshakeh; but a great deal of pride, malice, falsehood, and blasphemy. Hezekiah's nobles held their peace. There is a time to keep silence, as well as a time to speak; and there are those to whom to offer any thing religious or rational, is to cast pearls before swine. Their silence made Rabshakeh yet more proud and secure. It is often best to leave such persons to rail and blaspheme; a decided expression of abhorrence is the best testimony against them. The matter must be left to the Lord, who has all hearts in his hands, committing ourselves unto him in humble submission, believing hope, and fervent prayer.