25 And the multitude did see and hear and bear record; and they know that their record is true for they all of them did see and hear, every man for himself; and they were in number about two thousand and five hundred souls; and they did consist of men, women, and children. (3 Nephi 17:25)I find it interesting that the author says that we, "know that their record is true for they all of them did see and hear" (emphasis added). In other words, we can know that what happened was true because the people, and so many of them, actually saw and heard what was done. The truthfulness of the experience came because so many people were able to verify by themselves, and to testify that what was recorded actually did happen.
At another point in the story, Jesus spoke to the twelve disciples that He had chosen but the rest of the people did not hear what He had said and the author records that "therefore [the multitude] did not bear record; but the Disciples bare record that he gave them power to give the Holy Ghost. And I will show unto you hereafter that this record is true." (3 Ne. 18:37)
In this case only twelve people heard what Christ had said, and they were able to testify that they had been given power to give the Holy Ghost. But this was not enough, because it later had to be shown that not only what they had said was true, but that they actually had the power to give the Holy Ghost. In the next chapter this fact is established and those who witnessed it gave testimony to its veracity.
The common theme here is that whether or not some thing is true is established on the presence of witnesses that can all attest to what happened, including the doctrine that was taught and what Christ had said. The reason why I found this so interesting is that it stands in stark contrast to how many other Christian religions establish the veracity of their doctrine. Related to this idea, I once had a conversation with a friend of mine who is Christian, but is not Mormon, and he told me how he came to know that the Bible was true. The way he described it was that he had had an experience that allowed him to know it was true, but that experience could not be shared with or even conveyed to another person, because it was of "the spirit" and had no external, or provable manifestation.
When he told me this I thought it was very odd because that was directly opposite to everything I had ever known about how God and the Spirit works. As I had been frequently taught growing up, anyone could ask and find out for himself whether or not anything taught by the Church or in the scriptures is true. As one general authority, Elder Rex C. Reeve, put it:
We have this message today for each one of you—that the authority of God has been restored and his church has been reestablished in the earth. I testify to you of this truth! But you do not have to take my word for it; you can know for yourselves. Those who are seeking and will humble themselves and reach up to our Father in Heaven in prayer can know of a surety whether or not it is the truth. ("Feed My Sheep" Ensign Nov. 1980)This approach is diametrically opposed to that taken by so many others in Christianity that rely on unverifiable and unprovable assertions of truth. But as we can see from the passages from 3rd Nephi the truth of the doctrine and of special events such as the visitation of Christ to the Nephites is established by verifiable and witnessed events. More than this, as the quote from Elder Reeve shows, individual points of doctrine can be verified and anyone can come to a knowledge of its truthfulness.