Just as with the Transfiguration itself, the discussion preceding it is not mentioned in the Gospel of John. The other three, the Synoptic Gospels, are nearly identical (Matt. 16:28; Mk. 9:1; Lk. 9:27). The Lord had taken His disciples to Caesarea Philippi (Matt. 16:13), and He told them that some of them would see Him coming in His kingdom. About a week later, He chose Peter, James, and John to accompany Him to a secluded place on the mount (Matt. 17:1; Mk. 9:2; Lk. 9:28), where His Transfiguration occurred. Since He had said some of His disciples would see Him in His kingdom before they died, He could not have meant only John (who saw Him in his Patmos vision), for then He would have said "one of you." No, I believe that the Transfiguration was the fulfillment of what He had said the week before. The description of the Lord's appearance was basically the same as in John's vision (Matt. 17:2; Mk. 9:3; Lk. 9:29; Rev. 1:12-15). The some were Peter, James, and John.
After Jesus was transformed, Moses and Elijah appeared before them (Matt. 17:3; Mk. 9:4; Lk. 9:30). Some believe that since Elijah will appear prior to the Lord establishing His earthly kingdom (Mal. 4:5; Matt. 11:14), it will be Moses and Elijah who are the two witnesses in John's vision (Rev. 11:3-12). That makes sense since Moses represents the Law, and Elijah represents the Prophets, but the view that the two are Enoch and Elijah merits consideration as well. Enoch represents the Gentiles, and Elijah the Jews. Those who hold that it is Enoch instead of Moses say it is because of Hebrews 9:27, which many interpret to say everyone must die once. However, in light of the fact that the Rapture occurs prior to the two witnesses coming on the scene (Rev. 4:1), and apparently, living believers do not taste death at the Rapture of the Church (1 Thes. 4:16-17), only God knows who the two witnesses are.
We do, however, know why the two were there. Luke 9:31 tells us that they spoke of His death "which He should accomplish at Jerusalem." Since Jesus had already taught His disciples about His pending death (Matt. 16:21; Mk. 8:31; Lk. 9:22), it is obvious that it was He who was explaining His future death to Moses and Elijah. (Is it any wonder why Israel had no idea about God's Suffering Servant, when two of God's greatest Old Testament figures required special instruction concerning the Lamb of God's death?) Both had spoken of Israel's Messiah, Christ, King, but neither understood it would require Him to make "two trips" before He would come into His kingdom. The two had served the Lord long before the writing of Psalm 22, Isaiah 53, and Daniel 9.
In addition to all that, it is amazing John didn't record the event since he actually heard the voice of the Father (Matt. 17:5; Mk. 9:7; Lk. 9:35). He did not write of the Father's words at the baptism of Jesus either, but the other three Gospel writers did (Matt. 3:17; Mk. 1:11; Lk. 3:22). To those who deny the Trinity, this probably reinforces their error, but the Gospel of John uses the term "Father" in well over one hundred verses, and clearly teaches Jesus, Himself being God, was also with God (Jn. 1:1; 3:16; 17:1; etc.). In addition, He refers to God as "Father" at least twenty times in his epistles and Revelation.
I am not sure if God will have me write more on the Transfiguration, but we will see tomorrow, Lord willing.