Nephi's testimony is not merely a personal opinion. He is not guessing about what the Nephites are doing in secret. Nor is his testimony his own fabrication. His testimony is of the Lord. This is the kind of witness and testimony which is irrefutable. We would do well to follow his example in our own testimonies by acknowledging the source of spiritual knowledge. This will always strengthen the spirit and validity of the witness.
Orson Pratt
"I felt as though I was not qualified to stand before the people, and tell them that the Book of Mormon was a divine revelation, and that Joseph Smith was a prophet of God, unless I had a stronger testimony than that afforded by ancient prophets. However great my assurance might be, it seemed to me, that to know for myself, it required a witness independent of the testimony of others. I sought for this witness. I did not receive it immediately, but when the Lord saw the integrity of my heart and the anxiety of my mind--when He saw that I was willing to travel hundreds of miles for the sake of learning the principles of the truth, He gave me a testimony for myself, which conferred upon me the most perfect knowledge that Joseph Smith was a true prophet, and that this book, called the Book of Mormon, was in reality a Divine revelation, and that God had once more, in reality, spoken to the human family. What joy this knowledge gave me! No language that I am acquainted with could describe the sensations I experienced when I received a knowledge from Heaven of the truth of this work." (Journal of Discourses, vol. 12, p. 85)
Joseph F. Smith
"I received a testimony for myself from the Lord of this work, and that Joseph Smith was a Prophet of God before I ever saw him, while I resided in the state of New York, given in answer to prayer. I knew him in his lifetime and know him to have been a great, true man, and a servant of God." (Collected Discourses 1886-1898, ed. by Brian Stuy, vol. 5, Dec. 23, 1894)