Nephi had no trouble understanding Isaiah. This is reminiscent of Joseph Smith’s comment about the book of Revelation. He said, “the book of Revelation is one of the plainest books God ever caused to be written” (Teachings of the Prophet Joseph Smith, p. 290). It is easy for Nephi to say that Isaiah is not hard to understand and for Joseph Smith to say that the book of Revelation is one of the plainest books ever written, because they had both seen in vision the same things that Isaiah and John the Revelator saw. If we had the advantage of seeing in vision the same things seen by these great prophets, the interpretation would be a piece of cake.
Nevertheless, this is no excuse for us. The Lord has specifically commanded us to study Isaiah, a commandment I give unto you that ye search these things diligently; for great are the words of Isaiah (3 Nephi 23:1). We must seek the spirit of prophecy in order to understand Isaiah. The gift of prophecy is one of the gifts of the spirit (1 Cor 12:10). It is through this gift that Isaiah can be understood. We should seek this gift as with the other gifts of the spirit, seek ye earnestly the best gifts (DC 46:8).
John the Revelator further explained the spirit of prophecy, the testimony of Jesus is the spirit of prophecy (Rev 19:10). Some have mistakenly assumed that only a priesthood holder, ordained as a prophet of the Lord, could have this spirit. Nothing could be further from the truth. Every member should have a testimony of Jesus. Every member should have some portion of this spirit of prophecy. Without it, the things of the spirit cannot be understood (see DC 50:13-24). As our testimony of the Savior is strengthened, we will gain greater spiritual gifts until we become like Nephi, filled with the spirit of prophecy.
Hugh Nibley
“Remember, when they asked Joseph Smith, ’Are you a prophet of God?’ He said, ’Yes, and so is any other person who can bear a testimony of Jesus Christ.’ This can only be done by the spirit of prophecy.” (Ancient Documents and the Pearl of Great Price, lecture 21, p. 6)
Joseph Fielding Smith
“Revelation may be given to every member of the Church. The Prophet said that every man should be a prophet; that the testimony of Jesus is the spirit of prophecy. It is not only the privilege but the duty of each member of the Church to know the truth which will make him free. This he cannot know unless it is revealed to him. Moroni has promised every person who humbly and sincerely reads the Book of Mormon that he may know by revelation that it is true. The gift of the Holy Ghost is given to the members of the Church so that they may have the spirit of prophecy and revelation. Let it be understood however, that they will not receive revelation for the guidance of the Church. That gift is reserved for and vested in the man who holds the keys of the Priesthood. But the members of the Church are entitled to receive revelation which is needful for their progress and if they will hearken to the Spirit of truth and walk humbly before the Lord, they will not fall short of this spiritual guidance.” (Church History & Modern Revelation, vol. 4, p. 36)
The reason that Nephi and Joseph Smith had a far greater capacity to understand the prophecies of others was not because they were entitled to the spirit of prophecy and we are not. Rather, their ability comes from a more sure word of prophecy. As Peter said, We have also a more sure word of prophecy; whereunto ye do well that ye take heed, as unto a light that shineth in a dark place, until the day dawn, and the day star arise in your hearts (2 Pet 1:19). And, The more sure word of prophecy means a man’s knowing that he is sealed up unto eternal life, by revelation and the spirit of prophecy, through the power of the Holy Priesthood (DC 131:5).