This is my beloved Son. Listen to him. It is always easy to focus on, and marvel at the visual events that took place during Jesus’ transfiguration on the mountain (Jesus clothes becoming dazzling while and the appearance of Elijah and Moses) and not pay much attention to the voice from heaven that was heard. Even though Peter, James and John had by this time emerged as those closest to Jesus, for Mark they were still disciples (learners). Like the rest, they were still in the process of knowing who Jesus was. They were making two steps ahead and one step behind. Less than a week earlier, Peter had portrayed this clearly when he earned a rebuke from Jesus barely a second after confessing Jesus as the Christ. This lack of fully knowing and understanding Jesus prompted Jesus to give them a lecture about what was expected of them as Jesus’ followers. Six days later, the event of the transfiguration took place.
The disciples, and especially Peter, must still have been mulling over such things when Jesus got transfigured before them. They were understandably confused and frightened by what they were witnessing (remember they had not fully understood this man Jesus and what he was about). This is why the voice heard coming from the cloud that covered them was important: it was a voice that served to assure them (in case they were starting to have some doubts about this Jesus thing). Despite what they had heard from Jesus (suffering and dying), he was truly God’s beloved. As such, they were not to take a step back or forsake Jesus. He had words of eternal life, and men and women were to listen to him.