II. The same continued
But this exalted and highly-praised understanding, dearly-beloved, had also to be instructed on the mystery of Christ's lower substance, lest the Apostle's faith, being raised to the glory of confessing the Deity in Christ, should deem the reception of our weakness unworthy of the impassible God, and incongruous, and should believe the human nature to be so glorified in Him as to be incapable of suffering punishment, or being dissolved in death. And, therefore, when the Lord said that He must go to Jerusalem, and suffer many things from the elders and scribes and chief of the priests, and the third day rise again, the blessed Peter who, being illumined with light from above, was burning with the heat of his confession, rejected their mocking insults and the disgrace of the most cruel death, with, as he thought, a loyal and outspoken contempt, but was checked by a kindly rebuke from Jesus and animated with the desire to share His suffering.
For the Saviour's exhortation that followed, instilled and taught this, that they who wished to follow Him should deny themselves, and count the loss of temporal things as light in the hope of things eternal; because he alone could save his soul that did not fear to lose it for Christ. In order, therefore, that the Apostles might entertain this happy, constant courage with their whole heart, and have no tremblings about the harshness of taking up the cross, and that they might not be ashamed of the punishment of Christ, nor think what He endured disgraceful for themselves (for the bitterness of suffering was to be displayed without despite to His glorious power), Jesus took Peter and James and his brother John, and ascending a very high mountain with them apart, showed them the brightness of His glory; because, although they had recognised the majesty of God in Him, yet the power of His body, wherein His Deity was contained, they did not know.
And, therefore, rightly and significantly, had He promised that certain of the disciples standing by should not taste death till they saw “the Son of Man coming in His Kingdom,” that is, in the kingly brilliance which, as specially belonging to the nature of His assumed Manhood, He wished to be conspicuous to these three men. For the unspeakable and unapproachable vision of the Godhead Itself which is reserved till eternal life for the pure in heart, they could in no wise look upon and see while still surrounded with mortal flesh. The Lord displays His glory, therefore, before chosen witnesses, and invests that bodily shape which He shared with others with such splendour, that His face was like the sun's brightness and His garments equalled the whiteness of snow.
Source: Sermons (New Advent)