The Holy Ghost — also called the Spirit of the Lord, the Holy Spirit, and the Spirit of God — is the third member of the Godhead and a personage of spirit rather than of flesh and bones. Nephi, shown the Spirit of the Lord in vision, recorded that the Spirit was in the form of a man and spoke with him “as a man speaketh with another” (1 Nephi 11:11).
By the power of the Holy Ghost a person may know the truth of all things, and Moroni exhorts readers to ask God whether his record is true, promising that God will manifest the truth of it through the Holy Ghost (Moroni 10:4-5). The Holy Ghost bears record of the Father and the Son, and the Father gives the Holy Ghost to people because of Christ (3 Nephi 28:11). The Spirit of the Lord also guides and protects God’s servants: Alma and his brethren preached by the spirit of prophecy and revelation (Alma 43:2), and the Spirit of the Lord was with Samuel the Lamanite on the wall so that the stones and arrows thrown at him could not hit him (Helaman 16:2).
Receiving the Holy Ghost follows baptism by water. Nephi calls this the baptism of fire and of the Holy Ghost, given after one repents and witnesses to the Father a willingness to take Christ’s name, after which a person can speak with the tongue of angels (2 Nephi 31:13-14). Nephi explains the mechanism: after receiving the Holy Ghost, a person speaks with the tongue of angels because angels themselves speak by the power of the Holy Ghost — they speak the words of Christ (2 Nephi 32:2-3). Christ taught the Nephites the same sequence: those baptized with water would be baptized with fire and with the Holy Ghost and receive a remission of their sins (3 Nephi 12:1-2). King Benjamin taught that the natural man is an enemy to God and remains so forever unless he yields to the enticings of the Holy Spirit and puts off the natural man, becoming a saint through the atonement of Christ (Mosiah 3:19). Those sanctified by the Holy Ghost are made pure and spotless, abhor sin, and enter into the rest of the Lord (Alma 13:12).
Mormon, writing his abridgment of these records, did so that a knowledge of these things might come to the remnant of his people and to the Gentiles (Mormon 5:9). The remission of sins brings the visitation of the Holy Ghost, the Comforter, which fills a person with hope and perfect love (Moroni 8:26). The Father, the Son, and the Holy Ghost are described as one God, without end, in what Nephi calls the doctrine of Christ (2 Nephi 31:21). Moroni teaches that the gifts of God are many and are given by the manifestations of the Spirit of God to people, to profit them (Moroni 10:7-8).