Either in the deep. I understand it simply to mean Either above or below. He allows him an unrestricted choice of a miracle, to demand either what belongs to earth or what belongs to heaven. But perhaps in the word deep there is something still more emphatic; as if he had said, "It belongs to you to choose. God will immediately show that his dominion is higher than this world, and that it likewise extends to all depths, so that at his pleasure he can raise the dead from their graves." It was undoubtedly astonishing forbearance towards this wicked king and people of God, that not only did he patiently bear their distrust for a time, but so graciously condescended to them that he was willing to give them any pledge of his power which they chose. Yet he had in his eye not unbelievers only, but he intended likewise to provide for the benefit of the weak, in whom there was a seed of godliness; that they might be fully convinced that Isaiah did not speak at random, for he could easily give a proof of the power of God in confirmation of what he had said. The same goodness of God is now also displayed towards men, to whom he exercises such forbearance, when he might justly have been offended at them; for how shockingly do they insult God, when they doubt his truth? What do you leave to God, if you take that from him? And whatever may be our doubts, not only does he pardon us, but even aids our distrust, and not only by his word, but by adding miracles; and he exhibits them not only to believers, but also to the ungodly, which we may behold in this king. And if he was at that time so kind to strangers, what ought not his own people to expect from him?

Ask thee - Ask for "thyself;" ask a sign that shall be convincing to "thyself," since thou dost not fully credit the words of the prophet. It is evident that the words of the prophet had made no impression on the mind of Ahaz. God, therefore, proposes to him to ask any "proof or demonstration" which he might select; anything that would be an indication of divine power that should put what the prophet had said beyond doubt. Had Ahaz put confidence in God, he would have believed what the prophet said without miraculous proof. But he had no such confidence. 'The prophet, therefore, proposes that he should ask any miraculous demonstration that what he said would come to pass. This proposition was made, probably, not so much from respect to Ahaz as to leave him without excuse, and in order that "the people" might have the assurance that the city and kingdom were safe.
A sign - A demonstration that shall confirm the promise now made, and that shall be an evidence that Jerusalem shall be safe. The word used here, and translated "sign" - 'owt - אות 'ôth - means "a flag," or "a standard," Numbers 2:2; "a memorial or pledge" of a covenant, Genesis 17:11; any "pledge, token, or proof" of a divine mission, Judges 6:17; or a miracle performed in attestation of a divine promise or message. This is its sense here. That which Isaiah had spoken seemed highly improbable to Ahaz, and he asked him to seek a proof of it, if he doubted, by any prodigy or miracle. It was customary for miracles or prodigies to be exhibited on similar occasions; see Isaiah 38:7, where the shadow on the dial of this same Ahaz was carried backward ten degrees, in proof of what the prophet Isaiah had spoken; compare 1-Samuel 2:27-34; 1-Kings 13:1-3; Exodus 3:12; Judges 6:36-40. That the word here refers to some event which could be brought about only by divine power, is evident from the whole connection. No mere natural occurrence could have satisfied Ahaz, or convey to the people a demonstration of the truth of what the prophet was saying. And if the prophet had been unable or unwilling to give a miraculous sign, where is the fitness of the answer of Ahaz? How could he be regarded as in any way tempting God by asking it, unless it was something which God only could do? And how could the prophet bring the charge Isaiah 7:13, that he had not merely offended men, but God also? It is clear, therefore, that Isaiah was conscious that he was invested by God with the power of working a miracle, and that he proposed to perform any miracle which Ahaz should suggest that would serve to remove his doubts, and lead him to put confidence in God.
Ask it either in the depth - He gave him his choice of a miracle - any sign or wonder in heaven, or on earth - above or below; a miracle in the sky, or from beneath the earth. Many of the versions understand the expression 'the depth,' as referring to "the grave," or to the region of departed souls - "hades." So the Vulgate, Aquila, Symmachus. The Chaldee reads it, 'Seek that there may be a miracle to thee upon the earth, or a sign in the heavens.' The literal meaning of the Hebrew is, 'make low, ask for;' that is, ask for a sign below; obtain, by asking for thyself; a miracle that shall take place below. It may refer to the earth, or to the region under the earth, since it stands in contrast with that which is above. If it refers to the region under the earth, it means that Isaiah would raise the dead to life if Ahaz desired it; if to the earth, that any wonder or miracle that should take place in the elements - as a tempest, or earthquake - should be performed.
The height above - The heaven, or the sky. So the Pharisees desired to see a sign from heaven, Matthew 16:1.

In the depth "Go deep to the grave" - So Aquila, Symmachus, Theodotion, and the Vulgate.

Ask thee (i) a sign from the LORD thy God; ask it either in the depth, or in the height above.
(i) For the confirmation of this thing that your enemies will be destroyed and you preserved.

Ask thee a sign of the Lord thy God,.... For though Ahaz was a wicked man, yet the Lord was his God, as he was the God of the people of Israel in general, as a nation; and knowing his unbelief and diffidence of his word unto him, offers to confirm it by a sign or miracle:
ask it either in the depth, or in the height above, in earth, or in heaven: so the Targum,
"ask that a miracle may be done for thee upon earth, or that a sign may be shown thee in heaven;''
either that the earth might gape and open its mouth, as in the days of Moses; or that the sun might stand still, as in the times of Joshua; or that a dead man might be raised out of the depth of the earth; or that there might be some strange appearances in the heavens.

Ask thee--since thou dost not credit the prophet's words.
sign--a miraculous token to assure thee that God will fulfil His promise of saving Jerusalem (Isaiah 37:30; Isaiah 38:7-8). "Signs," facts then present or near at hand as pledges for the more distant future, are frequent in Isaiah.
ask . . . in . . . depth--literally, "Make deep . . . ask it," that is, Go to the depth of the earth or of Hades [Vulgate and LOWTH], or, Mount high for it (literally, "Make high"). So in Matthew 16:1. Signs in heaven are contrasted with the signs on earth and below it (raising the dead) which Jesus Christ had wrought (compare Romans 10:6-7). He offers Ahaz the widest limits within which to make his choice.