We should notice that while our missals tell us today’s Gospel is Matthew, 17: 1-8, it actually leaves off the first three words of verse one that state., "After six days."
Those words made a point that Matthew felt important. He was telling us that today’s incident was brought on by Our Lord’s announcement a week before that he was going to be put to death. Jesus had saddened himself and the Apostles by saying their happy days were drawing to a close. He told them he was headed up toward Jerusalem where he would be handed over to foreigners to be put to death.
This was so opposite to what the Apostles had been expecting that his words went over the heads of most of them. As well, they missed his dire warning that the Apostles too would need to take up crosses.
Those sad predictions might have sunk in a little with Peter, James, and John. So, a week later, when Jesus went up a mountain, seeking comfort from his Father, he brought them along.
On the mountain the Apostles had a heavenly experience that was really beyond words. Maatthew's Gospel account is just his effort at giving us an impression of how God consoled them on the mountain.
The Apostles had been sleeping, then a bright light woke them to the strangest of sights. The bottom of heaven seemed to be like a trampoline that stretched down just far enough to take in Jesus above them, and they saw him temporarily in heaven, transformed into a being in glory.
The Jews had a belief that two mortals were already in heaven. The buried Moses together with the whole of his grave had been taken up to heaven. As well, Elijah had been visibly taken up in a fiery chariot. Those two came over to chat with Jesus. Luke tells us they discussed the upcoming end to Our Lord’s mission on earth.
Then, as Peter told us in his Second Letter, they heard the Father calling Jesus his Son. The story is meant to do for us what it did for Jesus and for Peter, James, and John. It is meant to assure us that heaven is waiting for those who are faithful.