Literary: Nephi uses repetition to both emphasize a point, and to shift the focus of his discourse. He repeats the steps required to bring a person to baptism that he gave in the previous verse. This repetition serves to parallel and emphasize those steps. However, the repetition also serves here to allow Nephi to use the steps to emphasize not the baptism (the focus of the previous verse) but the gift of the Holy Ghost.
Scriptural: Nephi mentions that after the personal descension of the Holy Ghost, one “can speak with a new tongue, yea, even with the tongue of angels.” Certainly the gift of tongues is a gift of the spirit (1 Corinthians 12:10; Alma 9:21; Moroni 10:15-16) but it is not clear that this particular phrase refers to the “gift of tongues.” Nephi suggests that this “baptism of fire and of the Holy Ghost” will lead to a situation where one’s testimony is so firm, that should one afterward fall away “it would have been better for you that ye had not known me.” This suggests that something more than the gift of tongues is meant.
In this case, Nephi is speaking of a spiritual transformation - an alteration from the natural man to a spiritual man. Nephi is referring to what will later be termed a “mighty change” (Mosiah 5:2).