Question: Why did Nephi and Jacob (and the Savior) quote so much from the words of Isaiah?
Answer: The word Isaiah means “Jehovah is salvation.” Isaiah lived near Jerusalem from 760 to 700 B.C. Isaiah taught of Christ and of covenants made between God and Israel. Both Jacob and Nephi quote entire chapters of Isaiah and then give editorial commentary to illustrate the power of Isaiah’s prophecies
President Wilford Woodruff is quoted as saying, “Three fourths of Isaiah’s predictions related to the establishment of the kingdom of God in the latter days.” For this reason and more, Nephi and his brother Jacob, and the Savior, quoted extensively from the words of Isaiah.
Nephi quoted Isaiah because Isaiah speaks of the Jews of old, the Nephites, the latter days, and the coming forth of the Book of Mormon. After quoting thirteen chapters of Isaiah (2 Nephi 2-14) Nephi explains the reason he quoted from Isaiah: “Yes, and my soul delighteth in the words of Isaiah...in the days that the prophecies of Isaiah shall be fulfilled, men shall know of a surety, at the times when they shall come to pass. Wherefore, they are of worth unto the children of men...wherefore, for their good have I written them.” (2 Nephi 25:5, 7-8)
Jacob explained why he was asked by Nephi to teach his brethren the words of Isaiah. “And now, behold, I would speak unto you concerning things which are, and which are to come; wherefore, I will read you the words of Isaiah...And I speak unto you for your sakes, that ye may learn and glorify the name of your God. And now, the words which I shall read are they which Isaiah spake concerning all the house of Israel; wherefore, they may be likened unto you, for ye are of the house of Israel...” (2 Nephi 6:4-5)
The Savior quoted Isaiah about events that had been or would be fulfilled: “Verily, verily, I say unto you, thus hath the Father commanded me–that I should give unto this people this land for their inheritance. And then the words of the prophet Isaiah shall be fulfilled.” (3rd Nephi 16:16-17)
The Savior also taught, “Ye remember that I spake unto you, and said that when the words of Isaiah should be fulfilled–behold they are written, ye have them before you, therefore search them–And verily, verily I say until you, that when they shall be fulfilled, then is the fulfilling of the covenant which the Father hath made unto his people, O house of Israel.” (3 Nephi 20:11-12)
The Savior commanded, “And now, behold, I say unto you, that ye ought to search these things. Yea, a commandment I give unto you that ye search these things diligently; for great are the words of Isaiah.” (3 Nephi 23:1)
Source: 400 Questions and Answers About the Book of Mormon by Susan Easton Black, p. 101; Scriptures