Narrative: Nephi repeats the steps required to bring a person to baptism (v. 13), both to emphasize the importance of those steps and also to shift the focus of his discourse from baptism itself to the gift of the Holy Ghost.
Scripture: After a person receives the Holy Ghost, he or she “can speak with a new tongue, yea, even with the tongue of angels.” Although the gift of tongues is a gift of the spirit (1 Cor. 12:10, Alma 9:21, Moro. 10:15–16), Nephi does not seem to be referring to that gift. Instead, he seems to be referring to the state of those after the “baptism of fire and of the Holy Ghost.” In that state, their testimonies are so firm that, should they fall away, “it would have been better” if they had never “known [the Lord].”
Nephi is referring to what will later be termed a “mighty change” (Mosiah 5:2)—a transformation from the natural man to a spiritual man.