In each of the four gospels, in Acts, and in Romans, Isaiah 6:9-10 is quoted. The most complete example is in Acts, and occurs right at the end of the book. [Acts 28:25-31]
The account of Paul’s ministry ends rather abruptly, and many think that the original conclusion of the book of Acts is missing. Other references to Isaiah 6:9-10 are Matthew 13:14-15, Mark 4:12, Luke 8:10, John 12:40, and Romans 11:8.
In the following I focus on the next verse, in which the prophet Isaiah asked for how long the real meaning of prophecy will be hidden, and the answer he was given. On Isaiah 6:11 and 12, Princeton professor Joseph Addison Alexander commented: 
11. And I said, How long, Lord? And he said, Until that cities are desolate for want of an inhabitant, and houses for want of men, and the land shall be desolated, a waste, or utterly desolate. The spiritual death of the people should be followed by external desolation. The common explanation is no doubt the true one, that the Prophet asks how long the blindness of the people shall continue, and is told until it ruins them and drives them from their country. As the foregoing description is repeatedly applied in the New Testament to the Jews who were contemporary with our Saviour, the threatening must be equally extensive, and equivalent to saying that the land should be completely wasted, not at one time but repeatedly.
12. This verse continues the answer to the Prophet’s question in the verse preceding. And (until) Jehovah shall have put far off (removed to a distance) the men (or people of the country) and great (much or abundant) shall be that which is left (of unoccupied forsaken ground) in the midst of the land. This is little more than a repetition, in other words, of the declaration in the verse preceding. The terms of this verse may be applied to all the successive desolations of the country, not excepting that most extreme and remarkable of all which exists at the present moment.
Since Alexander’s time, a Jewish state has been established in Palestine. Many Jews have migrated there. But there has been no fulfillment of Isaiah 6:11-12. If the land was previously desolate, and without inhabitant, and is now inhabited by God’s people, wouldn’t Isaiah’s prophecy imply that the time when the meaning of his prophecies would be known had come?
Evidently, Isaiah must have referred to another land. The prophecy could refer to the land of promise in its spiritual sense. It can’t be the literal land, in the sense Alexander supposed above. The Jewish people now living in the modern Jewish state are no more enlightened about the Gospel, than the Jews of Rome were when Paul was there, and when the last verses of the book of Acts were written.
Alexander interpreted the land in a spiritual sense, in his comments on Isaiah 57:13, where he explained “he that putteth his trust in me shall possess the land” as having “a wider explanation, in which the possession of the land is an appointed symbol of the highest blessings which are in reserve for true believers here and hereafter.”  This is the spiritual meaning of the land; the land of Canaan was a type and shadow of the spiritual blessing promised to believers.
Ebenezer Henderson (1784-1858) understood the healing mentioned in Isaiah 6:10 as referring to spiritual conversion, and the forgiveness of sins. He wrote, “The healing referred to is the entire moral recovery which sinners experience on their conversion to God; and, as pardon is essential to such recovery, healing and forgiveness of sins came to be regarded by the Hebrews as synonymous.”  Henderson commented on the Hebrew terms used in verse 11: 
11. עַ֣ד אֲשֶׁר֩ אִם This accumulation is designed to give intensity to the statement, and thereby to intimate, in such connexion, the great length of time during which the obstinacy of the Jews should be evinced. Comp. Gen. xxviii. 15; Numb, xxxii. 17. תִּשָּׁאֶ֥ה שְׁמָמָֽה lit. be laid waste, a desolation, for "be utterly wasted."
Like many scholars, Otto Kaiser thought that the prophecy of Isaiah 6:9-11 referred to the Babylonian captivity, but the references in the New Testament show that the enlightenment that was promised when the land was utterly desolate had not come. He wrote: 
As the answer shows, the prophet’s question, ‘How long, O Lord?’ in v. 11a, which has its parallel in the lament, relates not so much to the duration of his activity, which is to advance the hardening of people’s hearts, as to the duration of that hardening. Here it is again clear that the narrator is looking beyond the lifetime of Isaiah to the catastrophe of 587. Because the so-called memorial introduced by ch. 6 ends in a word of hope (cf. 8:17), God’s answer envisages the end of the hardening and the possibility given with it of a new beginning to his history.
Johann Peter Lange thought that Isaiah’s prophecy meant there would be a complete desolation of the land. He wrote: 
5. Then said I —
Vers. 11-13. The announcement of the judgment of hardening in vers. 9, 10 sounds quite absolute. Yet the Prophet hears underneath all that it is not so intended. It is impossible that the Lord should quite and forever reject His people, and abrogate the promises given to the fathers. He asks, therefore, “How long, Lord?” (соmр. Ps. vi. 4; xc. 13; Hab. ii. 6). He would say: What are to be quantitively and qualitatively the limits of that judgment of hardening? The answer is: First there must be an entire desolation and depopulating of the land; and when at last still a tenth of the inhabitants is in the land, that tenth part also must be decimated till nothing is left but the stump of a root or stem. That shall then be the seed of a holy future. The meaning of the words is perfectly clear.
The construction is as follows: and still there is in it (the land) a tenth part, and this is again decimated—after the manner of or in resemblance to the terebinth and oak, in which, when felled, a stump remains, its stump (of the tenth) is holy seed. Therefore a stump always remains, and that suffices to guarantee a new life and a new glorious future. This has been steadily verified in the people Israel, both in a corporeal and spiritual respect. After every overthrow, yea, after the most fearful visitations, that aimed at the very extinction of the people, a stump or stem was still always left in the ground. This people is even not to be destroyed. There is nothing tougher than the life of this everlasting Jew. And in spiritual respects it is just the same. Though every knee seems to bow to the old or the new Baal, yet the Lord has preserved always a fragment (7,000 it is called, 1 Kings xix. 18) in faithfulness.
George Wöosung Wade (1858-1941) focused on the “perversity” of the Israelites that in his view was to result in their “extermination by death or exile.” He wrote: 
11. Until, etc. i.e. the nation’s perversity will be brought to an
end by nothing short of its extermination by death or exile: cf. xxii.
become utterly waste. Better (with the LXX. and Vulg.), be left waste …
12. removed men, etc. i.e. caused them to be deported by a foreign enemy: cf v. 13.
Robert Jamieson, A. R. Fausset and David Brown supposed the prophecy
was fulfilled by the Babylonian captivity or by the destruction of
Jerusalem in 70 AD. But if that were so, the Jews would not longer be
blinded to the Gospel. They stated: 
11. how long–will this wretched condition of the
nation being hardened to its destruction continue?
until– (Isa 5:9) –fulfilled primarily at the Babylonish captivity, and more fully at the dispersion under the Roman Titus.
12. (2Ki 25:21).
forsaking–abandonment of dwellings by their inhabitants (Jer 4:29).
The comments above demonstrate that blindness and hardness of heart that Isaiah’s prophecy described affects Gentiles, as well as the Jews. Two world wars in the 20th century, in which Christians fought and killed other Christians, illustrate that this is true. The people are estranged from the spiritual land of promise, that the literal land represents. Thus, they do not understand the message of Isaiah.
Only when the people recognize that they have been long removed from that land, the spiritual promised land that Canaan represents, which has remained desolate and without inhabitant, can Isaiah’s prophecies be fully understood.
1. Joseph Addison Alexander. Isaiah translated and explained. 1851. pp. 91-92.
2. J. A. Alexander, Isaiah translated and explained. Volume 2. John Wiley, NY. 1851. pp. 331-332.
3. Ebenezer Henderson. The book of the prophet Isaiah. London: Hamilton, Adams. 1857. p. 54.
5. Otto Kaiser. Isaiah 1-12: A Commentary. Westminster John Knox Press. 1983. p. 132.
6. Johann Peter Lange. A commentary on the Holy Scriptures: critical, doctrinal, and homiletical, Volume 11 . Scribner, 1878. p. 110.
7. George Wöosung Wade. The Book of the Prophet Isaiah: with introduction and notes. London: Methuen. 1911. p. 42.
8. Robert Jamieson, A. R. Fausset and David Brown. Commentary Critical and Explanatory on the Whole Bible. 1871.
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