According to Donald Parry, the expression "the spirit of prophecy" belongs almost exclusively to the Book of Mormon, being listed there more than twenty times, against one single attestation of the quote, or phrase, in the bible (found in Revelation 19:10). the definition of the phrase, as presented in scripture, has reference to the testimony of Jesus. So, that is to say, if you have a testimony of Jesus and are worthy, you have the spirit of prophecy. Joseph Smith understood this well, and frequently made reference to it in his sermons. For example, on one occasion, Joseph Smith explained:
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.
Nephi explained to his brethren that the prophecies of Isaiah "were manifest unto the prophet [Isaiah] by the voice of the Spirit; for by the Spirit are all things made known unto the prophets, which shall come upon the children of men according to the flesh" (1 Nephi 22:2). Add to this statement, a statement made by John Taylor: "these scriptures evidently show that the testimony of Jesus was the very principle, essence, and power of the spirit of prophecy whereby the ancient prophets were inspired." [Donald W. Parry, "Isaiah's Prophecies of Jesus Christ, Zion, and the Millennium," F.A.R.M.S., p. 15]