In the Christian life, we are called to live by faith, and not by sight. This is not, however, a blind faith, but a well-founded trust in the eye-witnesses of the truth of Christ, who now guide us to become His followers. Like columns that rest firmly on rock, they support authoritatively our faith: they are the Apostles Peter, James and John, to which it was granted, (as later it was for Paul), to stand before the majesty of the risen Christ, that which is hidden to human eyes. This is the experience described by the Gospel text we are looking at today, which tells us about the Transfiguration of Christ (Matthew 17:1-9). This experience confirms them in the basic statements of the Christian faith and allows them to transmit that message, a message unchanged in its power, reaching us today. In our Christian life, their experience allows us to continue trusting in Christ, His word and promises, inspite of difficulties, to the destination they had the preview.
See the sermon here