In both the Gospels of St. Matthew and St. Mark, the account of the Transfiguration immediately follows Jesus’ prediction of His passion and His stating the arduous demands of discipleship (see Mt 16:21-28; Mk 8:31-38).
We may assume that the Transfiguration not only clearly revealed Jesus’ divinity for a moment. He also allowed His glory to shine forth for the encouragement of the key apostles who were present: Peter, James and John.
Like so many incidents in Jesus’ earthly ministry, the Transfiguration was foreshadowed in the Old Testament. In this case, the type (the foreshadowing) is God’s revelation of himself to Moses on Mount Sinai. The “Ignatius Catholic Study Bible (Gospel of Matthew)” lists several parallels between the two epiphanies as recorded in Exodus and in St. Matthew’s Gospel.
Each takes place on a mountain (see Ex 24:13 and Mt 17:1).
Each occurs on a seventh day (Ex 24:16 and Mt 17:1).
Each shows forth the glory of God’s presence.
Each reports God speaking through a cloud (Ex 24:16 and Mt 17:5).
Moses and Elijah are the key representatives of the Law and the prophets. Their coming to talk with Jesus shows Him to be the fulfillment of God’s revelation to them.
Only Luke specifies the subject of their conversation with Jesus. Moses and Elijah “spoke of his [Jesus’] exodus that he was going to accomplish in Jerusalem” (Lk 9:31).
Note the word “accomplish.” It was important for the apostles to know that Jesus did not simply endure His passion; He accomplished it, by totally submitting himself to it.
A final note: Peter proposed building three booths, one each for Jesus, Moses and Elijah. He may have wished to extend their stay on the Mount of Transfiguration. But St. Mark, who was Peter’s protégé, simply tells us that Peter blurted out this proposal because “he hardly knew what to say, they were so terrified” (9:6).