With regard to Jesus's mission, the transfiguration was the prelude to His death. It was the crowning of the first part of His mission, that of realizing perfect life. Because of this crowning, He was now able to pass to the second part of His mission, that of atoning death. It will at once be seen how closely united these things are.
The death of Christ would have been of no avail for the redemption of the world had it not been preceded by His perfect life. To say this is not for one single moment to undervalue the death of Christ. Had the life not been perfect, the death would have been nothing more than the tragic end of an ordinary life, ordinary because conformed to the tendency and habit of the centuries, that of sin. But blessed be God, there had been no such conformity in the years that had preceded the Cross.
The transfiguration divided the ways. Amid the glory of that resplendent hour, the first part of His mission was ended. There was ushered in the second part, as He descended from the mountain, turning His back for the second time upon the light of heaven and taking His way to the Cross, passed into the darkness of death. Follow carefully the life of Jesus from that mount to the green hill outside the city wall. The one thought in His mind was that of His death, and of His Cross. May it not be said that after the mount He was eager for death? There was no drawing back, there was no flinching. He set His face towards Jerusalem, and it almost seems as though He were impatient of delay. With straight undeviating course, He passed from the mount of transfiguration to the Cross. Death was the goal, the Cross the throne. So the transfiguration came into the life of Jesus as the crowning of His humanity, and therefore His preparation for the death by which man is redeemed.
Adapted from The Crises of the Christ, Book IV, Chapter XVI, by G. Campbell Morgan.