[11] And the vision of all is become unto you as the words of a book that is sealed, which men deliver to one that is learned, saying, Read this, I pray thee: and he saith, I cannot; for it is sealed:

[12] And the book is delivered to him that is not learned, saying, Read this, I pray thee: and he saith, I am not learned.

[13] Wherefore the Lord said, Forasmuch as this people draw near me with their mouth, and with their lips do honour me, but have removed their heart far from me, and their fear toward me is taught by the precept of men:

[14] Therefore, behold, I will proceed to do a marvellous work among this people, even a marvellous work and a wonder: for the wisdom of their wise men shall perish, and the understanding of their prudent men shall be hid.

The untranslated gold plates of the Book of Mormon correspond to the book that is sealed. Martin Harris brought some examples of the reformed Egyptian writing to a professor at Columbia University. At first, the professor confirmed that the writing was ancient hieroglyphics. But when he was told that the plates were delivered by an angel, he recanted and said, “I can’t read this book, because it is sealed.” The plates remained in the hands of Joseph Smith (the unlearned man), who was unable to read them based on his own meager education (but he was able to translate them with the help of God). Also, see 2 Nephi 27:15-19.

2 Nephi 27

[15] But behold, it shall come to pass that the Lord God shall say unto him to whom he shall deliver the book: Take these words which are not sealed and deliver them to another, that he may show them unto the learned, saying: Read this, I pray thee. And the learned shall say: Bring hither the book, and I will read them.

[16] And now, because of the glory of the world and to get gain will they say this, and not for the glory of God.

[17] And the man shall say: I cannot bring the book, for it is sealed.

[18] Then shall the learned say: I cannot read it.

[19] Wherefore it shall come to pass, that the Lord God will deliver again the book and the words thereof to him that is not learned; and the man that is not learned shall say: I am not learned.

The Apostasy, which was a result of changes to what the Lord set up, and people worshipping in words only. The second half of verse 14 adds that the wisdom of the wise men would perish and human understanding would be hidden.

It is the restoration of the gospel, which brought an end to the period of the Apostasy. This phrase is mentioned many times in the Book of Mormon and also includes the coming forth of the Book of Mormon, which triggered the restoration of the gospel.

In 2 Nephi 25, Nephi comments on the words of Isaiah and how valuable they are to the House of Israel (1-5) and then mentions that Jerusalem was destroyed and that the people were carried away captive into Babylon but would eventually be restored to the land of their inheritance (9-11). 
 

Several years after the crucifixion of Jesus (in about 70 A.D.), Jerusalem was destroyed again, and sometime after that, the tribes of Israel were scattered. Finally, the Lord set His hand a second time to these people from their lost and fallen state by bringing forth the Book of Mormon and restoring the gospel. This was the marvelous work and wonder. See 2 Nephi 25:14-18.

2 Nephi 25

[14] And behold it shall come to pass that after the Messiah hath risen from the dead, and hath manifested himself unto his people, unto as many as will believe on his name, behold, Jerusalem shall be destroyed again; for wo unto them that fight against God and the people of his church.

[15] Wherefore, the Jews shall be scattered among all nations; yea, and also Babylon shall be destroyed; wherefore, the Jews shall be scattered by other nations.

[16] And after they have been scattered, and the Lord God hath scourged them by other nations for the space of many generations, yea, even down from generation to generation until they shall be persuaded to believe in Christ, the Son of God, and the atonement, which is infinite for all mankind — and when that day shall come that they shall believe in Christ, and worship the Father in his name, with pure hearts and clean hands, and look not forward any more for another Messiah, then, at that time, the day will come that it must needs be expedient that they should believe these things.

[17] And the Lord will set his hand again the second time to restore his people from their lost and fallen state. Wherefore, he will proceed to do a marvelous work and a wonder among the children of men.

[18] Wherefore, he shall bring forth his words unto them, which words shall judge them at the last day, for they shall be given them for the purpose of convincing them of the true Messiah, who was rejected by them; and unto the convincing of them that they need not look forward any more for a Messiah to come, for there should not any come, save it should be a false Messiah which should deceive the people; for there is save one Messiah spoken of by the prophets, and that Messiah is he who should be rejected of the Jews.