The location and significance of the Transfiguration Mount will be the subject of a future post.
The scholarly consensus recognizes that Matthew (16:28-17:9) copies Mark (9:1-9) here and Matthew (16:28) significantly changes Jesus' introductory saying from "the kingdom of God come in power" to "the Son of Man coming in His kingdom." Jesus' original saying reads:
"Truly I tell you, there are some standing here who will not taste death until they see the kingdom of God COME WITH POWER (Mark 9:1)."
The unusual specification of the time interval ("6 days later") between this saying and its fulfillment in the Transfiguration implies that the kingdom comes in power" through the Transfiguration. Jesus specifies that only "some [disciples] standing here" of the 12 who are with Him will actually see "the kingdom come with power" and "some" turns out to be Jesus' inner circle, Peter, James, and John. Moses and Elijah represent "all the prophets in the kingdom of God (Luke 13:28)." Moses appears as the forerunner of Jesus, "the prophet like Moses" prophesied in Deuteronomy 18:15-20. Elijah's return as the forerunner of the Messiah fulfills the prophecy of Malachi 3:5, but John the Baptist fulfills Elijah's earthly prophetic role (Matthew 17:10-12; Luke 1:17).
Peter celebrates his role as an eyewitness of the Transfiguration and picks up on its meaning as the glorified Jesus coming in kingdom power:
"We did not follow cleverly devised myths when we made known to you the POWER AND COMING of our Lord Jesus Christ, but we were EYEWITNESSES of His majesty. For He received honor and glory from God the Father when that voice was conveyed to Him by the Majestic Glory, saying, "This is my Son, the Beloved, in whom I am well pleased." We ourselves heard this voice while we were with Him on the holy mountain (2 Peter 1:16-18)."
In Vol. 3 of his magisterial New Testament Introduction (pp. 143-171). Donald Guthrie carefully considers and refutes all the arguments against 2 Peter's authenticity. The relevant issues are too intricate to detain us here.