Peter, James, and John might have been questioning their decision to follow Jesus. The more they learned, the more outrageous his predictions and teachings seemed. Even John the Baptist was having doubts. (And trouble sure seemed to follow this guy around. The authorities were not in the least amused by Jesus.)
So one day, Jesus invites these three up to the mountaintop. What happened next quelled all doubts. On this day Jesus' mission on Earth changed from itinerant preacher to Messiah; from wise philosopher to Savior of Mankind.
Light from Light. True God from True God. The Transfiguration.