Nephi, having obtained a testimony “by the power of the Holy Ghost,” could speak by the authority given by the same divine Person. Learning does not give authority to speak for God. Nor is a blameless life the same as divine authority. Nor a desire to speak. One of the early revelations through the Prophet Joseph Smith was addressed to his brother Hyrum on this subject. The Lord said:
“Behold, I command you, that you need not suppose that you are called to preach until you are called. Wait a little longer, until you shall have my word, my rock, my church, and my gospel, that you may know of a surety my doctrine. ... Seek not to declare my word, but first seek to obtain my word, and then shall your tongue be loosed. ... But now hold your peace, study my word which hath gone forth among the children of men [the Bible], and also study my word which shall come forth among the children of men, or that which is now translating,” [the Book of Mormon], (D. & C. 11:15-22).
In this revelation, the Lord says (v. 21) that when his tongue is loosed, “Then, if you desire, you shall have my Spirit and my word, yea, the power of God unto the convincing of men.” Here, too, we learn that it is the Holy Spirit that giveth authority to speak.