Luke here describes an incident in Christ’s life of which only Peter, James and John were witnesses. It took place on a mountain (Mount Tabor near Nazareth according to tradition). Some days previously Peter had confessed that Christ was the “Messiah of God.” Christ had then foretold his passion and death at the hands of his enemies, to be followed by his resurrection. He had encouraged his disciples not to fear sufferings but to carry their cross and so reach their glory as he was about to do.
*The Navarre Bible Commentary on the Gospel of Luke *notes the following:
By His transfiguration Jesus strengthens His disciples’ faith, revealing a trace of the glory His body will have after the Resurrection. He wants them to realize that His passion will not be the end but rather the route He will take to reach His glorification. “For a person to go straight along the road, he must have some knowledge of the end–just as an archer will not shoot an arrow straight unless he first sees the target …]. This is particularly necessary if the road is hard and rough, the going heavy, and the end delightful” (St. Thomas Aquinas, “Summa Theologiae”, III, q. 45, a. 1).
) before his Crucifixion. It’s just a thought, though.