Witnesses

The Law of Moses required two or three witnesses to convict someone of a crime. Also, commercial transactions and signing of documents of importance was done in the presence of witnesses. As a general rule, a single witness was not competent to attest or testify. (See e.g. Deuteronomy 19:15 and Isaiah 8:2). This was probably also the case for the Nephites, who followed the Law of Moses until the coming of Christ.

For instance, in Alma 9, Alma is charging an entire city with wickedness and prophesy of destruction. The people respond

2 Who art thou? Suppose ye that we shall believe the testimony of one man, although he should preach unto us that the earth should pass away?...
6 And they said: Who is God, that sendeth no more authority than one man among this people, to declare unto them the truth of such great and marvelous things? 

When Amulek enters the scene, the people react differently

And now, when Amulek had spoken these words the people began to be astonished, seeing there was more than one witness who testified of the things whereof they were accused, and also of the things which were to come, according to the spirit of prophecy which was in them. (Alma 10:12)

The ancient Israelite law of witnesses is evidently at play. Apparently, this does not only apply to legal matters but also spiritual matters. One of the reasons Nephi quotes Isaiah, is to call on him as an additional witness to prove that his testimony of the Savior is true.

2 And now I, Nephi, write more of the words of Isaiah, for my soul delighteth in his words. For I will liken his words unto my people, and I will send them forth unto all my children, for he verily saw my Redeemer, even as I have seen him.

3 And my brother, Jacob, also has seen him as I have seen him; wherefore, I will send their words forth unto my children to prove unto them that my words are true. Wherefore, by the words of three, God hath said, I will establish my word. Nevertheless, God sendeth more witnesses, and he proveth all his words. (2 Nephi 11)

To Nephi, God proves his words by calling multiple witnesses. In this case, three eye-witnesses (Isaiah, Nephi and Jacob) who all saw the Redeemer. According to John, we also have three witnesses in heaven.

For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one. (1 John 5:7)

The Greek word translated into "bear record" in the KJV means to witness, testify or give evidence. Similar expressions are found in the Book of Mormon, like in 3 Nephi 11:32

I bear record of the Father, and the Father beareth record of me, and the Holy Ghost beareth record of the Father and me

There is another interesting example of the word of God being proven by the law of witnesses. Consider Ether 5:3-4

3 And unto three shall they be shown by the power of God; wherefore they shall know of a surety that these things are true.

4 And in the mouth of three witnesses shall these things be established; and the testimony of three, and this work, in the which shall be shown forth the power of God and also his word, of which the Father, and the Son, and the Holy Ghost bear record—and all this shall stand as a testimony against the world at the last day.

These verses talk about the three witnesses. Verse 4 is a long, convoluted sentence. It starts off by restating the law of witnesses and refers specifically to "these things" which is contextualized by the preceding verses and refers to the Book of Mormon. After sorting this a bit:

• In this work (the coming forth of the Book of Mormon), the power and word of God will be shown forth
• There will be three witnesses to testify of "these things" (the Book of Mormon)
• The Father, Son and Holy Ghost all bear record of the word of God (as found in the Book of Mormon)
• "All this" (above) shall stand as a testimony against the world at the last day

When demonstrating God's power in bringing forth his words in the form of the Book of Mormon in these latter days, there are two sets of witnesses: One on earth (Oliver Cowdery, David Whitmer and Martin Harris) and one in heaven (the Father, the Son and the Holy Ghost). See this post for more context. This should tell us something about both the truthfulness and the importance of the Book of Mormon.