Matthew 17

 As we read concerning the events on the mount of Transfiguration, Elder Bruce R McConkie summarizes -

 (1) Jesus singled out Peter, James, and John from the rest of the Twelve; took them upon an unnamed mountain; there He was transfigured before them, and they beheld His glory. Testifying later, John said, “We beheld his glory, the glory as of the only begotten of the Father” (John 1:14); and Peter, speaking of the same event, said they “were eyewitnesses of his majesty” (2 Peter 1:16). 

(2) Peter, James, and John, were themselves “transfigured before him” (Teachings, p. 158), even as Moses, the Three Nephites, Joseph Smith, and many prophets of all ages have been transfigured, thus enabling them to entertain angels, see visions and comprehend the things of God (Mormon Doctrine, pp. 725–26). 

(3) Moses and Elijah—two ancient prophets who were translated and taken to heaven without tasting death, so they could return with tangible bodies on this very occasion, an occasion preceding the day of resurrection—appeared on the mountain; and they and Jesus gave the keys of the kingdom to Peter, James, and John (Teachings, p. 158). 

(4) John the Baptist, previously beheaded by Herod, apparently was also present. It may well be that other unnamed prophets, either coming as translated beings or as spirits from paradise, were also present. 

(5) Peter, James, and John saw in vision the transfiguration of the earth, that is, they saw it renewed and returned to its paradisiacal state—an event that is to take place at the Second Coming when the millennial era is ushered in (D&C 63:20–21; Mormon Doctrine, pp. 718–19). 

(6) It appears that Peter, James, and John received their own endowments while on the mountain (Doctrines of Salvation, vol. 2, p. 165). Peter says that while there, they “received from God the Father honour and glory,” seemingly bearing out this conclusion. It also appears that it was while on the mount that they received the more sure word of prophecy, it then being revealed to them that they were sealed up unto eternal life (2 Peter 1:16–19; D&C 131:5). 

(7) Apparently Jesus himself was strengthened and encouraged by Moses and Elijah so as to be prepared for the infinite sufferings and agony ahead of him in connection with working out the infinite and eternal atonement. (Jesus the Christ, p. 373.) Similar comfort had been given him by angelic visitants following his forty-day fast and its attendant temptations (Matthew 4:11), and an angel from heaven was yet to strengthen him when he would sweat great drops of blood in the Garden of Gethsemane (Luke 22:42–44). 

(8) Certainly the three chosen apostles were taught in plainness “of his death and also his resurrection” (JST Luke 9:31), teachings which would be of inestimable value to them in the trying days ahead. 

(9) It should also have been apparent to them that the old dispensations of the past had faded away, that the law (of which Moses was the symbol) and the prophets (of whom Elijah was the typifying representative) were subject to Him whom they were now commanded to hear. 

(10) Apparently God the Father, overshadowed and hidden by a cloud, was present on the mountain, although our Lord’s three associates, as far as the record stipulates, heard only his voice and did not see his form” (Doctrinal New Testament Commentary, Vol. 1, p. 399).

"And as they came down from the mountain, Jesus charged them, saying, Tell the vision to no man, until the Son of man be risen again from the dead."  (Matthew 17:9)

Even though Peter, James, and John may have shared this with their associates after the Savior was resurrected, it appears the gospel writers were very careful in sharing all the sacred events that happened there.