Preface
I will offer the perspective of one who believes the Book of Mormon. Those who do not believe the Book of Mormon will disagree with most of what I say, and may even take offense at my comments. My intent is not to offend you, but to share my beliefs as they relate to the OP's question.
The Church of Jesus Christ of Latter-day Saints does not teach a doctrine of scriptural inerrancy that is comparable to, say, the Chicago Statement on Biblical inerrancy.
The official doctrinal statement is:
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. (Articles of Faith 1:8)
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Translation
The Book of Mormon itself provides a direct answer to the question of authenticity -- Moroni described the future of the record he was burying as follows:
Written and sealed up, and hid up unto the Lord, that they might not
be destroyed—To come forth by the gift and power of God unto the
interpretation thereof—Sealed by the hand of Moroni, and hid up unto
the Lord, to come forth in due time by way of the Gentile—The
interpretation thereof by the gift of God (Title page of the Book of
Mormon).
The Three Witnesses to the Book of Mormon (including Oliver Cowdery, who served as scribe for most of the translation) bore the following testimony:
And we also know that they [the plates] have been translated by the gift and power
of God, for his voice hath declared it unto us; (see here)
What made the original record authoritative? It was given by inspiration of God.
What made the translated record authoritative? Same thing.
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Inspiration
Members of The Church of Jesus Christ of Latter-day Saints hold a somewhat different view of inspiration than is found elsewhere -- we believe God's words never cease (see Moses 1:4, Pearl of Great Price), and that He speaks as authoritatively today as He did in the past.
We believe that God works through small and simple things (see Alma 37:6-7) and can accomplish His works through the weak and simple things of the earth (see Doctrine & Covenants 1:23).
We do not hold that inspiration is a rare, inaccessible, or historically inerrant process, nor do we believe that the human element--the recipient--is removed from the equation. If a scriptural author miscounted the number of people present or made a mistake in arithmetic this does not annul the inspiration the person received while writing the record--it just means God got His work done through an imperfect person, and what they wrote was adequate for His purposes. If not so, why work through human efforts at all? Neither in the spoken nor written word is perfection necessary for revelation to be received & conveyed (Although the scriptures teach much about history this is not their fundamental purpose. They were given to teach doctrine. I am not the least bit concerned if occasionally they got a little of the history wrong).
Much of the time, the Holy Ghost impresses thoughts upon people's minds, and those faithful-yet-imperfect people put those thoughts into their own words. Is it possible that two people could receive the same inspiration and yet put it into slightly different words? Yes.
And yet, God is sufficiently powerful that He is capable of getting His work done through imperfect people.
My own experience with the process of receiving revelation is described here.
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Authority
Some reading the section above will be concerned that, if the Holy Ghost can reveal anything to any person, where is there any order or authority?
Latter-day Saints believe in the principle of stewardship, and that God reveals to people what is appropriate for their stewardship. E.g. a Bishop can receive revelation for his ward, a mother & father for their family, but only the Prophet can receive revelation for the world. We are counseled to be wary of false revelation, and that God will not reveal to one individual something that is contradictory to what He has revealed through His authorized spokesperson (see Doctrine & Covenants section 28).
One (of many) of the benefits to the structure given by God is that prophets, seers, and revelators provide guard rails against which we can check our own efforts to understand the truth, as we grow into the principle of revelation (The Church of Jesus Christ of Latter-day Saints sustains The First Presidency and the Quorum of the Twelve Apostles as prophets, seers, and revelators).
One way God's methods of revealing truth is described is that He uses 3 channels:
- The Priesthood Channel (authorized representatives holding authority for their area of stewardship)
- The Personal Channel (personal revelation through the Holy Ghost)
- The Creation (God created a universe that obeys laws and gave us minds that can test and discover those laws)
The information available through each of these channels will be in harmony with that of the others, if we have understood it properly.
The ultimate authority then is not scripture, but God Himself. The scriptures are authoritative because they came from God.
NT Wright (not a Latter-day Saint) observed the following, which was quoted in General Conference by apostle Jeffrey R. Holland:
The risen Jesus, at the end of Matthew’s Gospel, does not say, ‘All
authority in heaven and on earth is given to the books you are all
going to write,’ but [rather] ‘All authority in heaven and on earth is
given to me.’ (N. T. Wright, The Last Word: Beyond the Bible Wars to a
New Understanding of the Authority of Scripture (2005), xi)
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Original language
Again, it is not my purpose to give offense, but rather to explain a principle on which I differ from others when it comes to the focus on the original language of scripture.
It has at times been suggested that a record cannot be truly inspired scripture if it is not preserved in its original language (I'll call this "the Premise"). I'll cite 4 of the reasons why I do not hold this view:
It is nowhere taught in scripture
It commits the historical fallacy of presentism -- even the books of the Bible have not all been available in the original languages since the date they were composed (see discussion by Tresmontant in The Hebrew Christ p. 24) -- so if we in the 21st century adopt this principle, we are assuming the Bible was written for us right now but not for people of several centuries ago. I emphatically disagree.
We don't have 100% of the original Biblical texts as they were recorded in the autographs. This leads to the reductive argument called "the all or nothing trade-off" If inspiration is all or nothing, since we can only recreate ~99.5% of the original NT and for the OT the % is even lower, that would mean the Bible is not inspired. If we reject the all or nothing trade-off, we must conclude that only some portions of the autographs (the ones that survived) were inspired. To claim that all original texts of the Bible must be extant today because the texts were inspired is to argue in a circle. On either side of the all-or-nothing tradeoff we are left with a less-than-fully-inspired Bible. This difficulty is avoided if "the Premise" above not accepted.
That God revealed a principle to one person in one language in no way abridges His ability to reveal it to another person in another language. Human words themselves are not "the truth", but rather a vehicle for conveying the truth.
To borrow an idea from Papias of Hieropolis:
For I imagined that what was to be got from books was not so
profitable to me as what came from the living and abiding voice (see
here)
To modify the idea slightly, the living and abiding voice of the Holy Ghost is most profitable for "doctrine, for reproof, for correction, for instruction in righteousness" (see 2 Timothy 3:16). The inestimable value of the scriptures is that they were given by revelation through the Holy Ghost through God's appointed servants, and the scriptures given by inspiration will best be understood by inspiration.
But the natural man receiveth not the things of the Spirit of God: for
they are foolishness unto him: neither can he know them, because they
are spiritually discerned. (1 Cor. 2:14)
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Conclusion
To those who believe that God speaks as authoritatively today as He did in the past, truth from God is not limited to what was said at one time and place in the past.
Because the Book of Mormon was translated into English by the gift and power of God, it is as authoritative in English as it was in Reformed Egyptian.
My own personal conviction of the truthfulness of the Book of Mormon comes not through my study of history or languages (though I find both very interesting), but in the manner in which Moroni taught: by the power of the Holy Ghost. Although that testimony is substantiated by many arguments developed by the wisdom of men, its foundation is and must always be revelation from God.
Disclaimer: my comments are the product of my own study and do not constitute official statements by The Church of Jesus Christ of Latter-day Saints