For with stammering lips. Some supply, that "it is as if one should say;" but that is superfluous. I therefore view these words as relating to God, who became, as the Prophet tells us, a barbarian to a people without understanding. This reproof must have wounded them to the quick, because by their own fault they made God, who formed our tongues, to appear to be "a stammerer." He does not as yet threaten them, but lays the blame on their indolence, that they rendered the proclamation of heavenly doctrine a confused noise, because of their own accord they shut their eyes, and thus derived no advantage from it. Their infatuation, in not hearing God speaking to them, is compared by the Prophet to a prodigy.

For - This verse is to be understood as a response to what the complaining and dissatisfied people had said, as expressed in the previous verse. God says that he will teach them, but it should be by another tongue - a foreign language in a distant land. Since they refused to hearken to the messages which he sent to them, and which they regarded as adapted only to children, he would teach them in a manner that should be "much more" humiliating; he would make use of the barbarous language of foreigners to bring them to the true knowledge of God.
With stammering lips - The word which is used here is derived from a verb (לעג lâ‛âg), which means to speak unintelligibly: especially to speak in a foreign language, or to stammer; and then to mock, deride, laugh at, scorn (compare Isaiah 33:19; Proverbs 1:26; Proverbs 17:5; Psalm 2:4; Psalm 59:9; Job 22:19). Here it means in a foreign or barbarous tongue; and the sense is, that the lessons which God wished to teach would be conveyed to them through the language of foreigners - the Chaldeans. They should be removed to a distant land, and there, in hearing a strange speech, in living long among foreigners, they should learn the lesson which they refused to do when addressed by the prophets in their own land.

For with stammering (k) lips and another tongue will he speak to this people.
(k) Let one teach what he can, yet they will no more understand him, than if he spoke in a strange language.

For with stammering lips and another tongue will he speak to this people. Or "hath spoken" (s); as parents and nurses, in a lisping manner, and in a language and tone different from what they use in common, speak unto their children, accommodating themselves according to their capacities and weakness; and so it is a continuation of the method to be used in instructing the Jews, as being like children: or else these words are to be considered as a reason why, since they refused instruction in this plain, easy, and gentle manner, by the ministry of the prophets of the Lord, he would speak to them in a more severe and in a rougher manner in his providences, and bring a people against them of a strange language they understood not, and so should not be able to treat and make peace with them, and who would carry them captive into a strange land; which was fulfilled by bringing the Chaldean army upon them, Jeremiah 5:15 see 1-Corinthians 14:21 and afterwards the Romans.
(s) So Gataker.

For--rather, "Truly." This is Isaiah's reply to the scoffers: Your drunken questions shall be answered by the severe lessons from God conveyed through the Assyrians and Babylonians; the dialect of these, though Semitic, like the Hebrew, was so far different as to sound to the Jews like the speech of stammerers (compare Isaiah 33:19; Isaiah 36:11). To them who will not understand God will speak still more unintelligibly.

The prophet takes the ki ("for") out of their mouths, and carries it on in his own way. It was quite right that their ungodliness should show itself in such a way as this, for it would meet with an appropriate punishment. "For through men stammering in speech, and through a strange tongue, will He speak to this people. He who said to them, There is rest, give rest to weary ones, and there is refreshing! But they would not hear. Therefore the word of Jehovah becomes to them precept upon precept, precept upon precept, line upon line, line upon line, a little here, a little there, that they may go and stumble backwards, and be wrecked to pieces, and be snared and taken." Jehovah would speak to the scoffing people of stammering tongue a language of the same kind, since He would speak to them by a people that stammered in their estimation, i.e., who talked as barbarians (cf., βαρβαρίζειν and balbutire; see Isaiah 33:19, compared with Deuteronomy 28:49). The Assyrian Semitic had the same sound in the ear of an Israelite, as Low Saxon (a provincial dialect) in the ear of an educated German; in addition to which, it was plentifully mixed up with Iranian, and possibly also with Tatar elements. This people would practically interpret the will of Jehovah in its own patios to the despisers of the prophet. Jehovah had directed them, through His prophets, after the judgments which they had experienced with sufficient severity (Isaiah 1:5.), into the true way to rest and refreshing (Jeremiah 6:16), and had exhorted them to give rest to the nation, which had suffered so much under Ahaz through the calamities of war (2 Chron 28), and not to drag it into another way by goading it on to rise against Assyria, or impose a new burden in addition to the tribute to Assyria by purchasing the help of Egypt. But they would not hearken (אבוּא = אבוּ, Isaiah 30:15-16; Ges. 23, 3, Anm. 3). Their policy was a very different one from being still, or believing and waiting. And therefore the word of Jehovah, which they regarded as en endless series of trivial commands, would be turned in their case into an endless series of painful sufferings. To those who thought themselves so free, and lived so free, it would become a stone on which they would go to pieces, a net in which they would be snared, a trap in which they would be caught (compare Isaiah 8:14-15).

Another tongue - By people of a strange language, whom he shall bring among them, seeing they will not hear him speaking, by his prophets, in their own language.