JOEL - Joseph Smith taught that after one has received and cultivated the gift of the Holy Ghost, if he continues to hunger and thirst after righteousness and endures faithfully the trials and tests of this life, he will find his calling and election made sure—that is he will gain the assurance of eternal life. It will then be his privilege, based upon continued obedience and fidelity to the laws of the Lord-for this is an additional blessing-to receive what the Prophet called the second comforter, the personal presence of Christ the Lord. (TPJS, pp. 149-51.)
This includes all those who are married in the temple and who live by every word that proceedeth forth from the mouth of God. "They are sealed by the Holy Spirit of promise... unto the day of redemption." (D. & C. 132:26.) They have the unconditional guarantee of exaltation in the highest heaven of the celestial world; they receive the assurance of godhood. An exhortation to make one's "calling and election sure" is found in Peter's writings (2 Pet. 1:3-10), and is associated with the "more sure word of prophecy" (2 Pet. 1:16-19). Joseph Smith indicates that this was the case with ancient prophets such as Isaiah, Ezekiel, John, Paul and others (TPJS, p. 151).
“The more sure word of prophecy means a man’s knowing that he is sealed up unto eternal life, by revelation and the spirit of prophecy, through the power of the Holy Priesthood.” (D&C 131:5.)
Joseph Smith said, "There are two Comforters spoken of. One of these is the Holy Ghost, the same as given on the day of Pentecost, and that all saints receive after faith, repentance, and baptism." Then he explains that after a person's calling and election is made sure "it will be his privilege to receive the other Comforter, which the Lord hath promised the saints, as is recorded in the testimony of St. John, in the 14th chapter, from the 12th to the 27th verses." (Teachings, p. 149-150.) The Lord explains in the Doctrine and Covenants,
"Wherefore, I now send upon you another Comforter, even upon you my friends, that it may abide in your hearts, even the Holy Spirit of promise; which other Comforter is the same that I promised unto my disciples, as is recorded in the testimony of John. This Comforter is the promise which I give unto you of eternal life, even the glory of the celestial kingdom; which glory is that of the church of the Firstborn, even of God, the holiest of all, through Jesus Christ his Son." (D. & C. 88:3-5.)
The other comforter is Jesus Christ Himself who will appear to that person and assure him of his exaltation. There are only a few we know of who have received the second comforter; Joseph Smith(Joseph Smith History), Paul the apostle(Acts 7:55), and the brother of Jared(Ether 3:20, 25-26), to name a few.