From a homily by St. Leo the Great on the meaning of the Transfiguration:
“The Lord reveals his glory in the presence of chosen witnesses. His body is like that of the rest of mankind, but he makes it shine with such splendor that his face becomes like the sun in glory, and his garments as white as snow.
The great reason for this transfiguration was to remove the scandal of the cross from the hearts of his disciples, and to prevent the humiliation of his voluntary suffering from disturbing the faith of those who had witnessed the surpassing glory that lay concealed…he was also providing a firm foundation for the hope of holy Church. The whole body of Christ was to understand the kind of transformation that it would receive as his gift. The members of that body were to look forward to a share in that glory which first blazed out in Christ their head.
The Lord had himself spoken of this when he foretold the splendor of his coming: Then the just will shine like the sun in the kingdom of their Father. Saint Paul bore witness to this same truth when he said: I consider that the sufferings of the present time are not to be compared to the future glory that is to be revealed in us. In another place he says: When Christ, your life, is revealed, then you also will be revealed with him in glory…
The radiance of the transfiguration reveals clearly and unmistakably the one who had been promised by signs foretelling him under the veils of mystery. As Saint John says: The law was given through Moses, grace and truth came through Jesus Christ…
When it comes to obeying the commandments or enduring adversity, the words uttered by the Father should always echo in our ears: ‘This is my Son, the beloved, in whom I am well pleased; listen to him.’”