The voice spoke three times, and on the third time we are told that “the voice came, and did speak unto them marvelous words which cannot be uttered by man.” Once again, we must remember that Nephi was the high priest over the church. What was one important thing that the high priest could say that no one else could? It was the sacred name of Jehovah. In the temple, the high priest actually wore a little gold plate on his forehead that had the name of Jehovah on it to indicate that he had taken upon him the name of God. It could be read, but it could not be spoken except on the Day of Atonement. Even then, only the high priest could speak that name. Under Jewish tradition, the name of God had to be spoken ten times—a perfect number of times.
Perhaps the voice the people heard in this prison experience said, “I am.” That was the name that was sacred and could not be spoken. Perhaps the Lord explained, “I am Jehovah, I have done this, I have brought people out of bondage, I have brought you here.” If the Lord used his name, maybe even ten times, that would be something that could not be spoken.