Though some find the writings of Isaiah hard to understand, Nephi de-clared that "they are plain unto all those that are filled with the spirit of prophecy." (2 Nephi 25:4.)
What is the "spirit of prophecy" which, according to Nephi, is so key to understanding Isaiah?
The apostle John gave a definition of it while recording the revelation given to him while he was in exile on the isle of Patmos: "The testimony of Jesus is the spirit of prophecy." (Revelation 19:10.)
The Prophet Joseph Smith invoked this passage and reiterated more than once that the testimony of Jesus is, indeed, the spirit of prophecy.
For example, on Dec. 30, 1842, he taught: "If any person should ask me if I were a prophet, I should not deny it, as that would give me the lie; for, according to John, the testimony of Jesus is the spirit of prophecy; therefore if I profess to be a witness or a teacher, and have not the spirit of prophecy, which is the testimony of Jesus, I must be a false witness; but if I be a true teacher and witness, I must possess the spirit of prophecy, and that constitutes a prophet; and any man who says he is a teacher or a preacher of righteousness, and denies the spirit of prophecy, is a liar, and the truth is not in him; and by this key false teachers and imposters may be detected." (Documentary History of the Church: 5:215-216.)
On an earlier occasion, July 2, 1839, the Prophet said this in an address to the Quorum of the Twelve: "Salvation cannot come without revelation; it is in vain for anyone to minister without it. No man is a minister of Jesus Christ without being a prophet. No man can be a minister of Jesus Christ except he has the testimony of Jesus; and this is the spirit of prophecy. Whenever salvation has been administered, it has been by testimony." (DHC 3:389-390.)
It follows logically, then, that those who would understand Isaiah's teachings — and impart to others the saving truths therein — must seek and develop within themselves the testimony of Jesus.
