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Joseph Smith claimed "We believe the Bible to be the word of God as far as it has been translated correctly". However, he used the term "translation" rather loosely to refer to the entire process of transmission from the divine source. The idea that his translation of the Bible was done more with the intent to correct understanding than the text is reasonable and consistent with his other practices and expressed attitudes regarding the text of scripture.
The Book of Mormon passage (1 Ne 13:26-28) cited suggests that the corruption of Christian doctrine originated with censorship, and probably at a time even predating our oldest manuscripts. Not only was there a possible loss of books, epistles, and other manuscripts in the Roman persecutions, but Christians themselves may have taken authentic writings to be heretical or otherwise offensive and destroyed or censored them. Such a process would be mostly undetectable and the missing original doctrine unrecoverable by scholarly methods.
The Book of Mormon also claims that the Gentiles would "stumble" because of the loss of these teachings, or perhaps in other words, that they would misinterpret and misunderstand the part they did have. The history of heresies and doctrinal controversies in Christianity suggests that this misunderstanding began at a very early age. It certainly did not help matters when concepts and language which do not appear in the Bible were were imported from Greek philosophy and grafted into Christian theology, sometimes at variance with the plain and express meaning of the scriptures.
James E. Talmage, an early 20th century LDS scholar (also an apostle) in his work "The Great Apostasy" drew from standard sources on the early history of Christianity to document the various changes in Christian doctrine and practice, from an LDS point of view. This is still the most standard work on the subject. It is not quite authoritative but comes closer to it than anything else.
Upvote:1
The answer would be both. There is much evidences within the bible itself that things have been omitted.
1 Chr 29:29 Now the acts of David the king, first and last, behold, they are written in the book of Samuel the seer, and in the book of Nathan the prophet, and in the book of Gad the seer, (KJV)
Num. 21:14 Wherefore it is said in the book of the wars of the Lord, What he did in the Red sea, and in the brooks of Arnon, (KJV)
Jos 10:13, 1 Kng 11:41, 2 Chr 9:29 12:15 20:34 33:19, 1 Cor 5:9, and Jude 1:14 also include missing books we don't have today.
Joseph Smith translation does not cover missing books though, but simply re translates many of the ones we have. There are places though where there are full stories added in. For instance the book of Genesis has several verses added in by Joseph Smith including 15 verses in Genesis 14. The belief held by LDS members and the church is that things were changed, and taken away.
Within the LDS Bible Dictionary there is a whole segment on lost scriptures which can be found here https://www.lds.org/scriptures/bd/lost-books
A little clarification on what Joseph Smith translation includes seems important. it is many edits to verses, but are not included in the biblical text itself but in the form of footnotes, and in the Index in the back of the LDS printing of the KJV. It is believed to be a translation in a similar way the Book of Mormon was translated. It was done by the power of God.
Upvote:4
+1 for the great question! To begin as Members of the LDS Church "We believe the Bible to be the word of God as far as it is translated correctly; we also believe the Book of Mormon to be the word of God." (Article of Faith 8)
Throughout my response I will be referencing Plain and Precious Truths Restored, an article written in October of 2006.
There are multiple doctrines restored with The Book of Mormon. Just to list a few:
One example of a Joseph Smith Translation is Matthew 18:11
KJV: For the Son of man is come to save that which was lost.
JST of KJV: For the Son of man is come to save that which was lost, and to call sinners to
repentance; but these little ones have no need of repentance, and I will save them.
So in answer to your question, yes. We believe there have been doctrines which have been physically removed from The Bible either by an honest mistake in translation, or a deliberate omission.
-- I am out of time right now to go into more depth, but I would encourage you to read the article I referenced above.