All the three Synoptic Gospels record the narrative of the Transfiguration immediately following this prediction. There is no break in the narrative—no chapter or verse division in the Greek original—and furthermore all three mention the fact that the Transfiguration occurred about a week after this statement, implying that the event was the fulfillment of the prediction. The connection between the two sections of narrative seems to preclude the possibility that Jesus here referred to anything but the Transfiguration, which was a miniature demonstration of the kingdom of glory. And Peter understood it that way (2 Peter 1:16–18).
After 6 days, Jesus took Peter, James and John, up on the mountain apart by themselves, and He was transfigured before them. His clothes became shining exceeding white like snow and Elijah appeared to them with Moses and they were talking to Jesus.
Moses represented those who died and are resurrected (Jude 9). Elijah represented those who are translated without seeing death (2 Kings 2:11). God the Father came in a cloud and said, ‘This is my Beloved Son.’ What the disciples had was a miniature picture of the Second Coming. Jesus said I’m going to give you a preview, before you die, of my Kingdom coming. And that’s what happened on that mountain. By giving them a glimpse of His kingdom, the Lord wanted to comfort His disciples to endure the severe pain they were going to have during His crucifixion.
In His service,