2 Nephi 31

As Nephi teaches -

"...the doctrine of Christ..." (Book of Mormon | 2 Nephi 31:2)

He writes of one aspect of this doctrine -

"...he that is baptized in my name, to him will the Father give the Holy Ghost..." (Book of Mormon | 2 Nephi 31:12)

The "giving" of the Holy Ghost is a gift or the Gift of the Holy Ghost for when -

"...ye receive the Holy Ghost; yea, then cometh the baptism of fire and of the Holy Ghost; and then can ye speak with the tongue of angels..." (Book of Mormon | 2 Nephi 31:13)

Which tongue of angels is speaking with a testimony the words of Christ. The LDS Bible Dictionary defines the Holy Ghost as -

"The third member of the Godhead and, as the name implies, a personage of Spirit, not possessing a body of flesh and bones (D&C 130:21–22). The Holy Ghost has been manifest in every dispensation of the gospel since the beginning, being first made known to Adam (1 Ne. 10:17–22; Moses 6:51–68). The Holy Ghost is manifested to men on the earth both as the power of the Holy Ghost and as the gift of the Holy Ghost. The power can come upon one before baptism, and is the convincing witness that the gospel is true. It gives one a testimony of Jesus Christ and of his work and the work of his servants upon the earth. The gift can come only after proper and authorized baptism, and is conferred by the laying on of hands, as in Acts 8:12–25 and Moroni 2:1–3. The gift of the Holy Ghost is the right to have, whenever one is worthy, the companionship of the Holy Ghost. More powerful than that which is available before baptism, it acts as a cleansing agent to purify a person and sanctify him from all sin. Thus it is often spoken of as "fire" (Matt. 3:11; 2 Ne. 31:17; D&C 19:31). The manifestation on the day of Pentecost (Acts 2) was the gift of the Holy Ghost that came upon the Twelve, without which they were not ready for their ministries to the world.

For some reason not fully explained in the scriptures, the Holy Ghost did not operate in the fulness among the Jews during the years of Jesus' mortal sojourn (John 7:39; 16:7). Statements to the effect that the Holy Ghost did not come until after Jesus was resurrected must of necessity refer to that particular dispensation only, for it is abundantly clear that the Holy Ghost was operative in earlier dispensations. Furthermore, it has reference only to the gift of the Holy Ghost not being present, since the power of the Holy Ghost was operative during the ministries of John the Baptist and Jesus; otherwise no one would have received a testimony of the truths that these men taught (cf. Matt. 16:16–17; 1 Cor. 12:3). When a person speaks by the power of the Holy Ghost that same power carries a conviction of the truth into the heart of the hearer (2 Ne. 33:1). The Holy Ghost knows all things (D&C 35:19) and can lead one to know of future events (2 Pet. 1:21).

Other names that sometimes refer to the Holy Ghost are Holy Spirit, Spirit of God, Spirit of the Lord, Comforter, and Spirit." (Bible Dictionary | H Holy Ghost:Entry)

"For the gate by which ye should enter is repentance and baptism by water; and then cometh a remission of your sins by fire and by the Holy Ghost." (Book of Mormon | 2 Nephi 31:17)

Faith in Jesus Christ, sincere repentance with a change of heart, baptism, receiving the Gift of the Holy Ghost, and then enduring to the end is the Doctrine of Christ -

"...for ye have not come thus far save it were by the word of Christ with unshaken faith in him, relying wholly upon the merits of him who is mighty to save.
20 Wherefore, ye must press forward with a steadfastness in Christ, having a perfect brightness of hope, and a love of God and of all men. Wherefore, if ye shall press forward, feasting upon the word of Christ, and endure to the end, behold, thus saith the Father: Ye shall have eternal life." (Book of Mormon | 2 Nephi 31:19 - 20)

"...this is the way; and there is none other way nor name given under heaven whereby man can be saved in the kingdom of God. And now, behold, this is the doctrine of Christ, and the only and true doctrine of the Father, and of the Son, and of the Holy Ghost, which is one God, without end. Amen." (Book of Mormon | 2 Nephi 31:21)