A relatively recent addition to the Book of Mormon, the Introduction explains that “The Book of Mormon is a volume of holy scripture comparable to the Bible [and] “is a record of God’s dealings with the ancient inhabitants of the Americas and contains, as does the Bible, the fulness of the everlasting gospel.”
It is a fundamental “truth” of Mormonism that the plain and simple truths, the “fulness” of the gospel, were substantially lost after the death of the apostles. The religion of Joseph Smith was a restoration of those truths. The Book of Mormon itself says of the Bible:
1 Nephi 13
24 And the angel of the Lord said unto me: Thou hast beheld that the book [the Bible] proceeded forth from the mouth of a Jew; and when it proceeded forth from the mouth of a Jew it contained the fulness of the gospel of the Lord, of whom the twelve apostles bear record; and they bear record according to the truth which is in the Lamb of God…
26 And after they go forth by the hand of the twelve apostles of the Lamb, from the Jews unto the Gentiles, thou seest the formation of that great and abominable church, which is most abominable above all other churches; for behold, they have taken away from the gospel of the Lamb many parts which are plain and most precious; and also many covenants of the Lord have they taken away…
28 Wherefore, thou seest that after the book hath gone forth through the hands of the great and abominable church, that there are many plain and precious things taken away from the book, which is the book of the Lamb of God.
A promise is made later in the same chapter that these plain and precious truths, which had been lost through the corruption of the great and abominable church, would be restored:
35 For, behold, saith the Lamb: I will manifest myself unto thy seed, that they shall write many things which I shall minister unto them, which shall be plain and precious [The Book of Mormon]; and after thy seed shall be destroyed, and dwindle in unbelief, and also the seed of thy brethren, behold, these things shall be hid up, to come forth unto the Gentiles, by the gift and power of the Lamb [through Joseph Smith].
38 And it came to pass that I beheld the remnant of the seed of my brethren, and also the book of the Lamb of God, which had proceeded forth from the mouth of the Jew, that it came forth from the Gentiles unto the remnant of the seed of my brethren [the Lamanites, or American Indians].
39 And after it had come forth unto them I beheld other books, which came forth by the power of the Lamb, from the Gentiles unto them, unto the convincing of the Gentiles and the remnant of the seed of my brethren, and also the Jews who were scattered upon all the face of the earth, that the records of the prophets and of the twelve apostles of the Lamb are true.
40 And the angel spake unto me, saying: These last records [The Book of Mormon], which thou hast seen among the Gentiles, shall establish the truth of the first [The Bible], which are of the twelve apostles of the Lamb, and shall make known the plain and precious things which have been taken away from them; and shall make known to all kindreds, tongues, and people, that the Lamb of God is the Son of the Eternal Father, and the Savior of the world; and that all men must come unto him, or they cannot be saved. (words in [square brackets] and italics added)
The following is taken from the Encyclopedia of Mormonism under ‘J’ for Joseph Smith Translation of the Bible:
A somewhat parallel statement [to that in 1 Nephi 13] came to Joseph Smith in June 1830 while he was restoring a revelation received by Moses, declaring that many things would be taken "from the book" which Moses would write, but that the missing information would be restored through another prophet and thus be "had again" among those who believe (Moses 1:41). Latter-day Saints believe that the "other records" referred to include the Book of Mormon, the Doctrine and Covenants, the Pearl of Great Price, the JST, and other records still to come forth, and that the prophet divinely raised up to begin restoring the lost material is Joseph Smith (see Scriptures: Forthcoming Scriptures). In light of the foregoing statements, it is worth observing that the principal difficulty in the Bible apparently has been omissions. The remaining text may be generally correct in itself, but many important doctrinal items (resulting from the loss of a single word, a verse, a longer passage, or even whole books in some instances) are now missing. (Words in [square brackets] and italics added]
Now if the Bible contains the “fulness of the everlasting gospel” what exactly did Joseph restore? In the introduction to the Book of Mormon we have a message which declares that the Bible contains the fulness of the gospel while at same time declaring that the Book of Mormon is a restoration of the fulness of the gospel. Matters are further complicated by the fact that much of what we would today recognise as Mormonism is not to be found either in the Bible or the Book of Mormon.
The Mormon Church would claim that revelation is progressive in nature and that “further light and knowledge” is one of the benefits of having modern prophets. In other words further restoration work as described in the above quote. This leaves us with a picture of the fulness of the gospel being consistently available in the Bible, being restored through the Book of Mormon, though it was never missing, and being topped up by modern revelation, though we had the fulness of the gospel all along. How did the restoration of “plain and precious truths” come to be so confusing?