Of the various doctrines discussed in John 16-17, two are critical for Latter-day Saints to understand: the gift of the Holy Ghost and how to gain eternal life. These two chapters are the conclusion of the Savior’s mortal ministry to the twelve. The setting for these important teachings is the Last Supper.
The Gift of the Holy Ghost
Shortly after the supper was concluded, the Savior had warned the twelve that He would leave them (see John 14:1-3). But rather than question where He was going, the apostles sorrowed for their loss (John 16:6). The Savior promised that he would not leave them orphans (see John 14:18 footnote a) but would return to them again (John 14:18-23). But in the mean time, He would send them another Comforter, even the Holy Ghost.
In John 16, the Savior discussed the importance of the Holy Ghost in the lives of the twelve, as well as all of God’s children who receive the ordinance that initiates the gift of the Holy Ghost in their lives. At first he chided the twelve for not asking where the Savior was going (16:5). But then He gave them this promise: “Nevertheless I tell you the truth; It is expedient for you that I go away: for if I go not away, the Comforter will not come unto you; but if I depart, I will send him unto you” (16:7).
The Savior offered no explanation as to why the gift of the Holy Ghost had not been given the twelve. Certainly they must have received the ordinance of laying on of hands! But they had not experienced the actual reception and would not until after the Savior’s ascension to heaven (see Acts 1-2).
The twelve had received manifestations of the Holy Ghost (see Matt. 16:13-17). But receiving a manifestation of the Holy Ghost and receiving the gift of the Holy Ghost are two separate things. “We need to distinguish between a manifestation of the Holy Ghost and the gift of the Holy Ghost,” said Elder Dallin H. Oaks. “As men and women desire to believe, they develop faith in God. (See Alma 32:26–43.) When they have enough faith, they can receive a manifestation of the Holy Ghost.”
Generally a manifestation of the Holy Ghost is a witness of the truth of doctrines taught by the Lord’s emissaries (see Moroni 10:4-5). Manifestations lead the follower to baptism and confirmation. Citing the story of King Lamoni and his people, Elder Oaks continued, “Here we see the power and witness of the Holy Ghost poured out upon a woman and a man who had not yet been baptized. After they and their followers were converted by this witness, they were baptized and received the gift of the Holy Ghost. Then, as the scripture records, ‘they became a righteous people’ and ‘the Lord did begin to pour out his Spirit upon them.’ (Alma 19:35–36).” 
The gift of the Holy Ghost is an important phase of the Light of Christ that is made available to all who come unto Christ through baptism. In his masterful discourse on the Light of Christ, President Marion G. Romney taught: “There are three phases of the light of Christ that I want to mention. The first one is the light which enlighteneth every man that cometh into the world; The second phase is the gift of the Holy Ghost; And the third is the more sure word of prophecy.”  The first phase is given to all mankind. The second and third phases are only available to those who have received certain prescribed ordinances. The gift of the Holy Ghost is the first comforter while “the more sure word of prophecy” has reference to the second comforter of Christ himself.
It appears that though living with the Savior during his mortal ministry, the twelve were only under the influence of the first phase of the Light of Christ. It was essential that they progress to the second phase, or gift of the Holy Ghost. Then they would be prepared to receive Christ back again in the third phase.
The Savior taught the twelve, “Howbeit when he, the Spirit of truth, is come, he will guide you into all truth: for he shall not speak of himself; but whatsoever he shall hear, that shall he speak: and he will shew you things to come” (16:13). Without the gift of the Holy Ghost, understanding the doctrines of the kingdom is not fully possible.
Elder Keith B. McMullin taught: “The Holy Ghost is the third member of the Godhead, sent forth by God to reveal all needful things. He teaches and testifies with divine power and clarity. His witness may go unheard or unheeded, forsaken or denied, but it is never misunderstood. ‘The Holy Ghost is a revelator’ (Joseph Smith, Teachings of the Prophet Joseph Smith, sel. Joseph Fielding Smith , 328). That which is received of him has a more powerful effect upon the soul than anything else received in any other way. A millennium of experience through sight, sound, touch, taste, smell, and all the powers of the universe combined cannot approach the sublime and complete experience of one brief moment under the influence of the Holy Ghost.” 
Because the twelve were without the gift of the Holy Ghost, they did not fully comprehend the Savior or His mission. Consequently, they were not fully ready to fulfill the divine commission to take the gospel to all the world (see Acts 1:6-8).
Life Eternal is to Know God
After the Savior concluded His teachings to the twelve, He “lifted up his eyes to heaven” and offered what has become called “the great intercessory prayer.” He began by recognizing that “the hour is come” for Him to perform the great atoning sacrifice (17:1). He acknowledged that the purpose of the Savior’s sacrifice was to “give eternal life” to all those who would accept the fulness of the gospel of Jesus Christ (17:2).
But what is eternal life? How is it to be defined? President Ezra Taft Benson stated: “Eternal life is the quality of life enjoyed by our Heavenly Father. Those who fully comply with His commandments believe the promise that they will have this quality of life.”  Likewise, President Marion G. Romney explained: “Immortality connotes life without end. Eternal life, on the other hand, connotes quality of life—exaltation, the highest type of immortality, the kind of life enjoyed by God himself.” 
Therefore, eternal life is the same as godhood. President Kimball explained: “Life was to be in three segments or estates: pre‑mortal, mortal, and immortal. The third stage would incorporate exaltation–eternal life with godhood–for those who would fully magnify their mortal lives.” 
How is eternal life gained? The answer was given in the Savior’s prayer: “And this is life eternal, that they might know thee the only true God, and Jesus Christ, whom thou hast sent” (17:3).
In the great King Follett discourse, Joseph Smith discussed the meaning of this statement: “Here, then, is eternal life–to know the only wise and true God; and you have got to learn how to be Gods yourselves, and to be kings and priests to God, the same as all Gods have done before you, namely, by going from one small degree to another, and from a small capacity to a great one; from grace to grace, from exaltation to exaltation, until you attain to the resurrection of the dead, and are able to dwell in everlasting burnings, and to sit in glory, as do those who sit enthroned in everlasting power.” 
Coming to know God comes only through the atonement of Jesus Christ. The atonement of Jesus Christ brings man at one with God in two ways. First, it provides the means by which we are free from the eternal consequences of sin and thus we become reconciled with God. Second, by enabling us to become like God and thus “at one” with Him.
Oneness with God is more than just forgiven of sins. “To be redeemed,” Elder Russell M. Nelson explained, “is to be atoned–received in the close embrace of God with an expression not only of His forgiveness, but of our oneness of heart and mind.” 
In the great intercessory prayer, the Savior prayed for such oneness between God and all of his children who live in the world “but not of the world.” He prayed, “Neither pray I for these [the apostles] alone, but for them also which shall believe on me through their word; That they all may be one; as thou, Father, art in me, and I in thee, that they also may be one in us: that the world may believe that thou hast sent me.” Continuing he plead, “And the glory which thou gavest me I have given them; that they may be one, even as we are one: I in them, and thou in me, that they may be made perfect in one; and that the world may know that thou hast sent me, and hast loved them, as thou hast loved me” (17:20-23).
Gift of the Holy Ghost and Oneness With God
Combining the two main topics of John 16-17, we see that it is not possible to gain oneness with God without coming to understand the ways of God. And one cannot understand the ways of God without the gift of the Holy Ghost.
 Dallin H. Oaks, “Spiritual Gifts,” Ensign, Sept. 1986, p. 68.
 Marion G. Romney, “The Light of Christ,” Ensign, May 1977, p. 43.
 Keith B. McMullin, “Ye May Know,” Ensign, May 1996, p. 9.
 Ezra Taft Benson, Teachings of Ezra Taft Benson (Salt Lake City: Bookcraft, 1988), p.26.
 Marion G. Romney, “The Worth of Souls,” Ensign, Nov. 1978, p.14.
 Spencer W. Kimball, The Miracle of Forgiveness (Salt Lake City: Bookcraft, 1969), p.4‑5
 Joseph Smith, Teachings of the Prophet Joseph Smith. (Salt Lake City: Deseret Book Press, 1938), p. 346.
 Russell M. Nelson, “The Atonement,” Ensign, Nov. 1996, p.34.