What does Luke 9:36 mean?

Peter, James, and John followed Jesus up a mountain and fell asleep. When they awake, Jesus is talking with Moses and Elijah. Then a cloud envelopes them all and God the Father speaks, saying, "This is my Son, my Chosen One; listen to him!" (Luke 9:28–35). Suddenly, the cloud is gone, and Moses and Elijah are nowhere to be found (Mark 9:8).

Jesus tells them to remain silent about what they had witnessed until His resurrection (Matthew 17:9; Mark 9:9). It's easy to understand why. The last time Jesus told the disciples He was going to die, Peter rebuked Him (Matthew 16:21–23). Before long, Jesus will tell the disciples again that he is going to be killed in Jerusalem and they still won't understand (Luke 9:44–45). No one in that era fully grasped that Jesus the Messiah must die for the sins of the world and be resurrected on the third day. After Jesus fed the thousands, some of the men were so enthusiastic they tried to force Jesus to be king (John 6:15, 26). If the crowds know what had happened on that mountain, they might revolt against the Roman government, leading to the deaths of thousands.

Peter, James, and John remain silent "in those days," but they obviously tell people or else Luke wouldn't have known. Peter wrote about it, reminding his audience how "they" had told them before (2 Peter 1:16–18). John wrote, "And we have seen and testify that the Father has sent his Son to be the Savior of the world" (1 John 4:14). When it occurred, the event was for Peter, James, and John, but Luke includes it to give Theophilus assurance, as well, "that [he] may have certainty concerning the things [he has] been taught" (Luke 1:4).