The “marvelous work and a wonder” doctrine comes from Isaiah, 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….And in that day shall the deaf hear the words of the book, and the eyes of the blind shall see out of obscurity, and out of darkness. The meek also shall increase their joy in the LORD, and the poor among men shall rejoice in the Holy One of Israel (Isa 29:14,18-19). What is marvelous and a wonder is the work the Lord performs in the last days. Verse 18 above shows that the Book of Mormon plays a role in the process but is not the “marvelous work and a wonder.” The phrase means that the Lord will build a kingdom in the latter days using the simple and meek of the earth. He will gather the tribes of Israel by a great missionary effort that will be successful because of the Book of Mormon and other latter-day scriptures. LeGrand Richards explains how crucial the ministry of Joseph Smith was to this process.
"Joseph Smith, or any other man, could not have obtained all this information by reading the Bible or studying all the books that have ever been written. It came from God. It is exactly what Isaiah promised the Lord would do when conditions upon the earth should become as Joseph Smith found them when he went into the woods to pray for light as to which of all the churches he should join….
“It truly is a marvelous work and a wonder. Can you conceive of anything that could be more marvelous or more wonderful?” (A Marvelous Work and a Wonder, pp. 411)
Heber J. Grant
“Truly a great and marvelous work has come forth and been proclaimed in every land and in every clime all over the wide world. The gospel of Jesus Christ has been restored--by a personal visitation of the apostles Peter, James and John, laying their hands upon the heads of Oliver Cowdery and the Prophet Joseph Smith and ordaining them to the apostleship; by a personal visitation of John the Baptist, who baptized the Savior, laying his hands upon Oliver Cowdery and Joseph Smith and ordaining them to the Aaronic, or the lesser priesthood--the gospel is again restored to the earth, with the power and authority that existed in the days of the Savior. Millions of dollars in money have been expended for erecting and maintaining temples wherein ordinances are performed for the salvation of those who have died without a knowledge of the gospel. All these things bear witness of the inspiration of God to that man Joseph Smith, when he delivered this statement in a revelation to his father, that a great and a marvelous work was about to come forth among the children of men.” (Conference Report, Apr. 1920, p. 4)