Jesus chose Peter, James and John to witness his transfiguration. The Apostles' faith was a growing faith that at times it was firm and strong; other times it was a wavering faith. They all made the full commitment not before, but after the Resurrection of Jesus. Peter became the first leader of the early church and was crucified upside down during the reign of Nero. James and his brother John were known as the sons of Zebedee. The pair earned a nickname "Sons of Thunder". James was the first of the twelve to be martyred for his faith in A.D. 44. John preferred to call himself 'the disciple whom Jesus loved'. He outlived all of the Apostles, dying not in martyrdom but of ripe age in exiled at Patmos. We enjoy the benefit of the doubts from the Apostles because each time they questioned about the faith, Jesus explained to them and showed them compassion and mercy and told them 'believe and doubt no more' Jn 20,27 Mat 21,21.
Like the Apostles, our faith is a growing faith. In following Jesus we sometimes feel strongly committed and other times we have some reservations. The Apostles had some doubts and learnt more, we too, doubt and learn. In the Bible the chosen ones like Mary, Joseph, the Apostles, Moses and Elijah, had a specific mission and there were little explanations or criteria explaining why they were chosen. It remained a mystery.
Apart from the call at the baptism, that is to live a life of active faith; to share the gifts of stewardship; and to bear witness for Jesus, each of us has numerous mysterious calls to serve Jesus in our daily lives and we don't know why God chose us to be in it. Some of the calls we embrace with joy; others we struggle hard to carry our cross to follow. The mysterious call comes from 'listen to him' in faith and trust even we don't understand. God is in control every moment of our lives; including natural events. Thunders and clouds and lightning were created for a purpose. The Apostles were 'overcome with fear'. 'Stand up' he said 'Do not be afraid' v 7-8. Jesus touched them and healed them. On the way down Jesus told them about his Passion that will happen before the glorification. We, too, each of us have our own passion. The transfiguration revealed a better life given after our earthly lives ended. Moses died 1200 years and Elijah died 700 years before the birth of Jesus and yet both of them were present at the Transfiguration. They are truly alive with God.
The Transfiguration revealed Jesus' glorification to the Apostles but for the Apostles it was their personal experience of the faith journey. They saw it and heart it but would not able to understand. The journey of faith provides no scientific facts but can make sense in prayers and contemplations.