The transfiguration is an event, which appears in all the Synoptic Gospels, but each narrates it differently. In Mark, it follows after the instructions that Jesus gives to the disciples at Caesarea Philippi and after six days. The event is a confirmation by God of the fact that Jesus is indeed Messiah, beloved Son. Most think that the reason for the choice of Elijah and Moses is that the Jews considered them as being alive in the presence of God. Jesus is superior even to these figures.
In Mk the order is Elijah and Moses. In Matthew, the order is Moses and Elijah (so Luke) to emphasize the two personalities of the OT who received revelation on Mount Sinai (Ex 19,33-34; 1Kgs 19,9-13) and personify the Law and the prophets. While in Mt Jesus is the New Moses and Luke emphasizes the approaching passion, Mark sees in the transfiguration the glorious manifestation of the hidden Messiah. Briefly the disciples experience the heavenly quality of Jesus. Jesus is no less Messiah when his Messianic glory is hidden in the passion, than he is at the Transfiguration.
Elijah was regarded as the prophet who would come before the Lord (Malachi 3,24-25; 4,5) as his messenger. Jesus’ reply in John suggests that Elijah has indeed come in John the Baptist is an indication that he is the Lord.
There are times in our lives when everything goes according to plan and at those times it is easy to see that God is on our side. However, when we are faced with trials and when things do not work out, as we want them to, then the transfiguration is a reminder to us that even when carrying our cross we are still beloved by God.