From The Gospel of Mark, by Mary Healy, commenting on Mark 9:2-10
The Transfiguration, like Jesus’ baptism (Mark 1:11), is a Trinitarian event, with the Holy Spirit’s presence now symbolized by the cloud rather than a dove. Just as at the baptism, the heavenly Father gives audible testimony to his beloved Son. At the baptism God had addressed Jesus himself; now he speaks to the disciples about Jesus, revealing a status that far exceeds that of Moses and Elijah.
This testimony to Jesus (here and at his baptism) is the only word the Father is recorded as saying in the Gospels, since Jesus is the fullness of all that he has to say to humanity.
The command to Listen to him recalls Moses’ promise that God would one day raise up “a prophet like me . . . from among your own kinsmen; to him you shall listen” (Deut 18:15). The disciples are to listen to everything Jesus has to say, but especially, in the context of the conversation that has just transpired (Mark 8:31–38), his prophecy about his messianic suffering and its implications for them. They have been shown a glimpse of the road far ahead: if they listen carefully and obey his commands all the way to the cross, their destiny will be joined to his, and they too will one day be transfigured with divine glory.
At the pinnacle of this experience the disciples suddenly find themselves with Jesus alone. Moses and Elijah have already accomplished their tasks, but Jesus must now complete the Father’s plan by going to the cross alone. His own life and mission will be the fulfillment that transcends all that took place in the Old Testament.