This is my Son: Except for the introductory declaration of “Jesus Christ, the Son of God” (1:1), Mark reveals the identity of Jesus as “Son of God” at his baptism, at his transfiguration, and after his death on the cross. At his baptism, Jesus sees the heavens being torn open and the Spirit descending upon him like a dove. And a voice comes from the heavens, “You are my beloved Son; with you I am well pleased” (1:10-11). After this, Jesus begins a ministry of power through exorcisms, healings, and miracles.
However, Mark reverses the Jesus-of-power in the second half of the Gospel. When Peter declares Jesus to be the Messiah (cf 8:27), Jesus teaches that the Messiah will be rejected, made to suffer and die, and resurrected. Mark then begins to demonstrate that discipleship means powerlessness. He needs to confirm this direction in the Gospel with a type of baptismal scene. The transfiguration accomplishes this.
From the cloud comes a voice, “This is my beloved Son. Listen to him” (v 8). The transfiguration serves to confirm the new direction of powerlessness in the Gospel. It also points beyond the suffering and death of Jesus to his resurrection, his glory. Mark is interested in a theme which is best stated in this way: real power is found in powerlessness. When Jesus hangs in powerless death on the cross, then he has attained the power of God. For as soon as he is dead, a centurion declares, “Truly this man was the Son of God!” (15:39).