Jesus had just promised the disciples that faithful identification with Him would be rewarded when He returned in glory (Luke 9:23-26).  Until then, everything He said had been fulfilled ... but now they were effectively staking their whole lives on this one promise ... how could they be sure?  The Lord, who knows everything, knew that they needed assurance in a way they could understand.  He took Peter, James and John to pray with Him up a mountain.  Although the disciples did not really know how to engage in prayer, and almost fell asleep, the changed appearance of Jesus made them fully alert.

Although they had never seen Moses or Elijah, the two men were instantly recognisable alongside Jesus.  The law and the prophets had done their work and now Jesus was seen to have fulfilled the ministry of them both (Matthew 5:17).  But their conversation was not about their own Old Covenant ministry, but about Jesus’ forthcoming New Covenant work on the cross (Luke 22:20).    

This event was so memorable that Peter used it to comfort and strengthen churches under persecution.  2 Peter 1:16-18 says, "For we did not follow cleverly devised stories when we told you about the coming of our Lord Jesus Christ in power, but we were eye-witnesses of his majesty.  He received honour and glory from God the Father when the voice came to him from the Majestic Glory, saying, ‘This is my Son, whom I love; with him I am well pleased.’  We ourselves heard this voice that came from heaven when we were with him on the sacred mountain.”

Many religions encourage faithfulness in their followers by promising good in the future.  But how does anyone know that those promises can be fulfilled?  Jesus showed that He could keep all His promises.  He spoke of glory, and then showed himself in glory.  Moses and Elijah, who had already prophesied about Jesus, now prophesied Him leaving the world as Saviour (in the same way that God led the Israelites out of Egypt to a new life).  Jesus not only spoke of new life but raised three people from the dead before He was also raised.  The suffering of Jesus was vindicated by the resurrection - a great encouragement for the suffering church in the years to come.  In short, Jesus' words are fully trustworthy.  We, who believe in Jesus, have every reason to be completely confident that everything He said will be seen. And we have every reason to recommend Jesus to our friends and family who have no such certainty.