Nephi first gave us his personal testimony and witness of the principles. Having done so, now he adds the testimony and promise of Christ. Christ's promise and covenant are slightly different than Nephi's formula. But the two are nevertheless in complete harmony.
The "voice of the Son" declares to Nephi, and Nephi testifies to us, that "after ye have repented of you sins" and you have "witnessed unto the Father that ye are willing to keep [Christ's] commandments" by receiving "baptism of water" and then have received "baptism of fire and the Holy Ghost" you will speak with "a new tongue."
How can a man speak with a "new tongue?" What does "a new tongue" mean?
Think of Isaiah's meeting with the Lord in the Temple. He confessed how unworthy his speech had been, and how much regret he felt at having been a man of "unclean lips." (Isa. 6: 5.) His lips were unclean because of the low, mean, vulgar and unworthy things that occupied daily conversation. Or, as Joseph put it: "How vain and trifling have been our spirits, our conferences, our councils, our meetings, our private as well as public conversations—too low, too mean, too vulgar, too condescending for the dignified characters of the called and chosen of God, according to the purposes of His will, from before the foundation of the world!" (Letter from Liberty Jail.)
To speak with a new tongue is to speak worthily of sacred things. It is to correctly weigh the truth of a matter, know by the power of the Spirit that what is said is true and in conformity with God's will and then to speak it. It is to render sacred the vessel by the things it holds.
To speak with a new tongue is to be able to speak with the tongue of an angel because you have become an angel; or a companion of angels anyway. It is to elevate your thoughts, and then what proceeds forth from your mouth, because of what is in your thoughts. It is to reveal truth by the things you are authorized or commissioned to speak. It is to have a right to speak in the name of the Lord by His consent, His authority, His will. It is to "know, nothing doubting" that He is your Lord. (Ether 3: 19.) It is to say, without hypocrisy, without guile, without hesitation and in truth, that the power of salvation is found in Christ and that you are His. That He has entrusted to you words of life, and that salvation can be found only in Him and His words. It is to have the Word of God within you.
Can an angel fall from grace? Only by being cast out of heaven. (2 Nephi 2: 17.) When an angel falls he becomes a devil. For these it would be better if they had never known Christ, for they have decided to crucify Him anew. Because after having had the Holy Spirit make great things known unto them they have turned against the Lord by their knowing rebellion against Him. (D&C 76: 35.) They are sons of perdition, and the heavens weep over them. (D&C 76: 26, 31-32.) These are they who know the battle is and always has been the Lord's, and they either align themselves with Him or against Him.
You cannot speak with the tongue of angels without having knowledge of certain things given you. The clarity with which you can declare truth is distinct from what others say or claim to know. Light and truth, which is intelligence or the glory of God (D&C 93: 36), is not a mystery but an understood and appreciated experience where darkness has fled and God's own glory has been upon you. (Moses 1: 11.)
This is what the Gospel of Christ was intended to confer. Not just belief, or faith, but knowledge and understanding. The journey back to God's presence was always the outcome intended by the Gospel. The Gospel message is and always has been that you should receive further light and knowledge by conversing with the Lord through the veil. Not through an intermediary, but in your own behalf, face to face.
The entirety of the process may be reduced to just a few words: You are intended to receive baptism of fire and the Holy Ghost, which purges you from all sin. After being made clean, every whit, which is suggested by "fire" then through the instrumentality of the "Holy Ghost" which dwells within you you may be brought into remembrance of all things.
These then are the words of both Nephi and Christ. They agree. They are the two witnesses of this doctrine and truth. Therefore, it is so.