Recall the prophecy of Daniel:

Daniel 7:13-14:
I beheld, therefore, in the vision of the night, and lo, one like the Son of man came with the clouds of heaven, and he came even to the ancient of days: and they presented him before him. And he gave him power, and glory, and a kingdom: and all peoples, tribes, and tongues shall serve him: his power is an everlasting power that shall not be taken away: and his kingdom that shall not be destroyed.

Today we recall when Moses, representing the Law, and Elias (Elijah), representing the Prophets -- two men who had special visions of God -- appear with Jesus on on Mt. Tabor (Matthew 17, Mark 9, Luke 9). There the Apostles see the Divine Uncreated Light shine forth from Our Lord, Who'd told them previously that He must die and be resurrected.

Matthew 17:1-8
And after six days Jesus taketh unto him Peter and James, and John his brother, and bringeth them up into a high mountain apart: And he was transfigured before them. And his face did shine as the sun: and his garments became white as snow. And behold there appeared to them Moses and Elias talking with him.

And Peter answering, said to Jesus: Lord, it is good for us to be here: if thou wilt, let us make here three tabernacles, one for thee, and one for Moses, and one for Elias.

And as he was yet speaking, behold a bright cloud overshadowed them. And lo, a voice out of the cloud, saying: This is my beloved Son, in whom I am well pleased: hear ye him.

And the disciples hearing, fell upon their face, and were very much afraid. And Jesus came and touched them: and said to them, Arise, and fear not. And they lifting up their eyes saw no one but only Jesus. And as they came down from the mountain, Jesus charged them, saying: Tell the vision to no man, till the Son of man be risen from the dead.

Christ, as the Temple Who would be raised up three days after 'it' was torn down, shows that He is, indeed, He in Whom the glory dwells. As the Creed says, "Deum de Deo, Lumen de lumine, Deum verum de Deo vero" (God from God, Light from Light, true God of true God).

Of significance to this Feast is what it reveals about true Judaism. From the Catholic Encyclopedia:

False Judaism had rejected the Messias, and now true Judaism, represented by Moses and Elias, the Law and the Prophets, recognized and adored Him, while for the second time God the Father proclaimed Him His only-begotten and well-loved Son.

St. John Chrysostom writes more about the appearance of Moses and Elias in his Homilies on the Gospel of Matthew:

But wherefore doth He also bring forward Moses and Elias? One might mention many reasons. And first of all this: because the multitudes said He was, some Elias, some Jeremias, some one of the old prophets, He brings the leaders of His choir, that they might see the difference even hereby between the servants and the Lord; and that Peter was rightly commended for confessing Him Son of God.

But besides that, one may mention another reason also: that because men were continually accusing Him of transgressing the law, and accounting Him to be a blasphemer, as appropriating to Himself a glory which belonged not to Him, even the Father's, and were saying, "This Man is not of God, because He keepeth not the Sabbath day;" and again, "For a good work we stone Thee not, but for blasphemy, and because that Thou, being a man, makest Thyself God:"that both the charges might be shown to spring from envy, and He be proved not liable to either; and that neither is His conduct a transgression of the law, nor His calling Himself equal to the Father an appropriation of glory not His own; He brings forward them who had shone out in each of these respects: Moses, because he gave the law, and the Jews might infer that he would not have overlooked its being trampled on, as they supposed, nor have shown respect to the transgressor of it, and the enemy of its founder: Elias too for his part was jealous for the glory of God, and were any man an adversary of God, and calling himself God, making himself equal to the Father, while he was not what he said, and had no right to do so; he was not the person to stand by, and hearken unto him.

And one may mention another reason also, with those which have been spoken of. Of what kind then is it? To inform them that He hath power both of death and life, is ruler both above and beneath. For this cause He brings forward both him that had died, and him that never yet suffered this.

But the fifth motive, (for it is a fifth, besides those that have been mentioned), even the evangelist himself hath revealed. Now what was this? To show the glory of the Cross, and to console Peter and the others in their dread of the Passion, and to raise up their minds. Since having come, they by no means held their peace, but "spake," it is said, "of the glory which He was to accomplish at Jerusalem;" that is, of the passion, and the cross; for so they call it always.

St. John Chrystom, in a continuation of the homily above, explains the deepest lesson of the Transfiguration:

But if we will, we also shall behold Christ, not as they [SS. Peter, James, and John] then on the mount, but in far greater brightness. For not thus shall He come hereafter. For whereas then, to spare His disciples, He discovered so much only of His brightness as they were able to bear; hereafter He shall come in the very glory of the Father, not with Moses and Elias only, but with the infinite host of the angels, with the archangels, with the cherubim, with those infinite tribes, not having a cloud over His head, but even heaven itself being folded up.

For as it is with the judges; when they judge publicly, the attendants drawing back the curtains show them to all; even so then likewise all men shall see Him sitting, and all the human race shall stand by, and He will make answers to them by Himself; and to some He will say, "Come, ye blessed of my Father; for I was an hungered, and ye gave me meat; "to others," Well done, thou good and faithful servant, thou hast been faithful over a few things, I will set thee over many things.

And again passing an opposite sentence, to some He will answer, "Depart into the everlasting fire, that is prepared for the devil and his angels,"and to others, "O thou wicked and slothful servants."And some He will "cut asunder," and "deliver to the tormentors;" but others He will command to "be bound hand and foot, and cast into outer darkness? And after the axe the furnace will follow; and all out of the net, that is east away, will fall therein.

"Then shall the righteous shine forth as the sun; "or rather more than the sun. But so much is said, not because their light is to be so much and no more, but since we know no other star brighter than this, He chose by the known example to set forth the future brightness of the saints.

Since on the mount too, when He says, "He did shine as the sun," for the same cause did He so speak. For that the comparison did not come up to His light, the apostles showed by falling down. For had the brightness not been unalloyed, but comparable to the sun; they would not have fallen, but would easily have borne it.

The righteous therefore will shine as the sun, and more than the sun in that time; but the sinners shall suffer all extremities. Then will there be no need of records, proofs, witnesses. For He who judges is Himself all, both witness, and proof, and judge. For He knows all things exactly; "For all things are naked and opened unto His eyes."

No man will there appear rich or poor, mighty or weak, wise or unwise, bond or free; but these masks will be dashed in pieces, and the inquiry will be into their works only. For if in our courts, when any one is tried for usurpation, or murder, whatever he may be, whether governor, or consul, or what you will, all these dignities fleet away, and he that is convicted suffers the utmost penalty; much more will it be so there.

Therefore that this may not be so, let us lay aside our filthy garments, let us put on the armor of light, and the glory of God will wrap us around.

See also the Commemoration of the Baptism of Our Lord Jesus Christ.

 
Reading

Sermon 51
by Pope St. Leo the Great (d. A.D. 461)

I.

The Gospel lesson, dearly-beloved, which has reached the inner hearing of our minds through our bodily ears, calls us to the understanding of a great mystery, to which we shall by the help of God's grace the better attain, if we turn our attention to what is narrated just before. The Saviour of mankind, Jesus Christ, in founding that faith, which recalls the wicked to righteousness and the dead to life, used to instruct His disciples by admonitory teaching and by miraculous acts to the end that He, the Christ, might be believed to be at once the Only-begotten of God and the Son of Man. For the one without the other was of no avail to salvation, and it was equally dangerous to have believed the Lord Jesus Christ to be either only God without manhood, or only man without Godhead, since both had equally to be confessed, because just as true manhood existed in His Godhead, so true Godhead existed in His Manhood.

To strengthen, therefore, their most wholesome knowledge of this belief, the Lord had asked His disciples, among the various opinions of others, what they themselves believed, or thought about Him: whereat the Apostle Peter, by the revelation of the most High Father passing beyond things corporeal and surmounting things human by the eyes of his mind, saw Him to be Son of the living God, and acknowledged the glory of the Godhead, because he looked not at the substance of His flesh and blood alone; and with this lofty faith Christ was so well pleased that he received the fulness of blessing, and was endued with the holy firmness of the inviolable Rock on which the Church should be built and conquer the gates of hell and the laws of death, so that, in loosing or binding the petitions of any whatsoever, only that should be ratified in heaven which had been settled by the judgment of Peter.

II.

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 flyings 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 tilt 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.

III.

And in this Transfiguration the foremost object was to remove the offence of the cross from the disciple's heart, and to prevent their faith being disturbed by the humiliation of His voluntary Passion by revealing to them the excellence of His hidden dignity. But with no less foresight, the foundation was laid of the Holy Church's hope, that the whole body of Christ might realize the character of the change which it would have to receive, and that the members might promise themselves a share in that honour which had already shone forth in their Head. About which the Lord bad Himself said, when He spoke of the majesty of His coming, "Then shall the righteous shine as the sun in their Father's Kingdom," whilst the blessed Apostle Paul bears witness to the self-same thing, and says: "for I reckon that the sufferings of this thee are not worthy to be compared with the future glory which shall be revealed in us:" and again, "for ye are dead, and your life is hid with Christ in God. For when Christ our life shall appear, then shall ye also appear with Him in glory."But to confirm the Apostles and assist them to all knowledge, still further instruction was conveyed by that miracle.

IV.

The significance of the appearance of Moses and Elias. For Moses and Elias, that is the Law and the Prophets, appeared talking with the Lord; that in the presence of those five men might most truly be fulfilled what was said: "In two or three witnesses stands every word." What more stable, what more steadfast than this word, in the proclamation of which the trumpet of the Old and of the New Testament joins, and the documentary evidence of the ancient witnesses combine with the teaching of the Gospel? For the pages of both covenants corroborate each other, and He Whom under the veil of mysteries the types that went before had promised, is displayed clearly and conspicously by the splendour of the present glory. Because, as says the blessed John, "the law was given through Moses: but grace and truth came through Jesus Christ," in Whom is fulfilled both the promise of prophetic figures and the purpose of the legal ordinances: for He both teaches the truth of prophecy by His presence, and renders the commands possible through grace.

V.

The Apostle Peter, therefore, being excited by the revelation of these mysteries, despising things mundane and scorning things earthly, was seized with a sort of frenzied craving for the things eternal, and being filled with rapture at the whole vision, desired to make his abode with Jesus in the place where he had been blessed with the manifestation of His glory. Whence also he says, "Lord, it is good for us to be here: if thou wilt let us make three tabernacles, one for Thee, one for Moses, and one for Elias."

But to this proposal the Lord made no answer, signifying that what he wanted was not indeed; wicked, but contrary to the Divine order: since the world could not be saved, except; by Christ's death, and by the Lord's example the faithful were called upon to believe that, although there ought not to be any doubt about the promises of happiness, yet we should understand that amidst the trials of this life we must ask for the power of endurance rather than the glory, because the joyousness of reigning cannot precede the times of suffering.

VI.

And so "while He was yet speaking, behold a bright cloud overshadowed them, and behold a voice out of the cloud, saying, "This is My beloved Son, in whom I am well pleased; hear ye Him." The Father was indeed present in the Son, and in the Lord's brightness, which He had tempered to the disciples' sight, the Father's Essence was not separated from the Only-begotten: but, in order to emphasize the two-fold personality, as the effulgence of the Son's body displayed the Son to their sight, so the Father's voice from out the cloud announced the Father to their hearing. And when this voice was heard, "the disciples fell upon their faces, and were sore afraid," trembling at the majesty, not only of the Father, but also of the Son: for they now had a deeper insight into the undivided Deity of Both: and in their fear they did not separate the One from the Other, because they doubted not in their faith. That was a wide and manifold testimony, therefore, and contained a fuller meaning than struck the ear. For when the Father said, "This is My beloved Son, in Whom, etc.," was it not clearly meant, "This is My Son," Whose it is to be eternally from Me and with Me? because the Begetter is not anterior to the Begotten, nor the Begotten posterior to the Begetter. "This is My Son," Who is separated from Me, neither by Godhead, nor by power, nor by eternity.

"This is My Son," not adopted, but true-born, not created from another source, but begotten of Me: nor yet made like Me from another nature, but born equal to Me of My nature.

"This is My Son," "through Whom all things were made, and without Whom was nothing made" because all things that I do He doth in like manner: and whatever I perform, He performs with Me inseparably and without difference: for the Son is in the Father and the Father in the Son, and Our Unity is never divided: and though I am One Who begot, and He the Other Whom I begot, yet is it wrong for you to think anything of Him which is not possible of Me.

"This is My Son," Who sought not by grasping, and seized not in greediness, that equality with Me which He has, but remaining in the form of My glory, that He might carry out Our common plan for the restoration of mankind, He lowered the unchangeable Godhead even to the form of a slave.

VII.

"Here ye Him," therefore, unhesitatingly, in Whom I am throughout well pleased, and by Whose preaching I am manifested, by Whose humiliation I am glorified; because He is "the Truth and the Life," He is My "Power and Wisdom."

"Hear ye Him," Whom the mysteries of the Law have foretold, Whom the mouths of prophets have sung.

"Hear ye Him," Who redeems the world by His blood, Who binds the devil, and carries off his chattels, Who destroys the bond of sin, and the compact of the transgression.

Hear ye Him, Who opens the way to heaven, and by the punishment of the cross prepares for you the steps of ascent to the Kingdom? Why tremble ye at being redeemed? why fear ye to be healed of your wounds? Let that happen which Christ wills and I will. Cast away all fleshly fear, and arm yourselves with faithful constancy; for it is unworthy that ye should fear in the Saviour's Passion what by His good gift ye shall not have to fear even at your own end.

VIII.

The Father's words have a universal application to the whole Church. These things, dearly-beloved, were said not for their profit only, who heard them with their own ears, but in these three Apostles the whole Church has learnt all that their eyes saw and their ears heard.

Let all men's faith then be established, according to the preaching of the most holy Gospel, and let no one be ashamed of Christ's Cross, through which the world was redeemed. And let not any one fear to suffer for righteousness' sake, or doubt of the fulfilment of the promises, for this reason, that through toil we pass to rest and through death to life; since all the weakness of our humility was assumed by Him, in Whom, if we abide in the acknowledgment and love of Him, we conquer as He conquered, and receive what he promised, because, whether to the performance of His commands or to the endurance of adversities, the Father's fore-announcing voice should always be sounding in our ears, saying, "This is My beloved Son, in Whom I am well pleased; hear ye Him:" Who liveth and reigneth, with the Father and the Holy Ghost, for ever and ever. Amen.