I answer that, Christ wished to be transfigured in order to show men His glory, and to arouse them to desire it. Now men are brought to the glory of eternal beatitude by Christ—not only those who lived after Him, but also those who preceded Him; therefore, when He was approaching His Passion, both \”the multitude that followed\” and those \”that went before, cried saying: \’Hosanna,\’\” as related Mt. 21:9, beseeching Him, as it were, to save them. Consequently it was fitting that witnesses should be present from among those who preceded Him—namely, Moses and Elias—and from those who followed after Him—namely, Peter, James, and John—that \”in the mouth of two or three witnesses\” this word might stand.
By His transfiguration Christ manifested to His disciples the glory of His body, which belongs to men only. It was therefore fitting that He should choose men and not angels as witnesses.
St. Jerome says, on Mt. 17:3: \”Observe that when the Scribes and Pharisees asked for a sign from heaven, He refused to give one; whereas here in order to increase the apostles\’ faith, He gives a sign from heaven, Elijah coming down from where he had ascended, and Moses arising from the nether world.\” This is not to be understood as though the soul of Moses was reunited to his body, but that his soul appeared through an assumed body, just as the angels do. But Elijah appeared in his own body, not that he was brought down from the empyrean heaven, but from some place on high where he was taken up in the fiery chariot.
St. John Chrysostom says, on Mt. 17:3: \”Moses and Elijah are brought forward for many reasons.\” 1st, \”because the multitude said He was Elijah or Jeremiah or one of the prophets, He brings the leaders of the prophets with Him; that hereby at least they might see the difference between the servants and their Lord.\” 2nd, \” … that Moses gave the Law … while Elijah … was jealous for the glory of God.\” Therefore by appearing together with Christ, they show how falsely the Jews \”accused Him of transgressing the Law, and of blasphemously appropriating to Himself the glory of God.\” 3rd, \”to show that He has the power of death and life, and that He is the judge of the dead and the living; by bringing with Him Moses who had died, and Elias who still lived.\” 4th, because, as Luke says (9:31), \”they spoke\” with Him \”of His passage that He should accomplish in Jerusalem,\” i.e. of His Passion and death. Therefore, \”in order to strengthen the hearts of His disciples with a view to this,\” He sets before them those who had exposed themselves to death for God\’s sake: since Moses braved death in opposing Pharaoh, and Elijah in opposing Ahab. 5th, that \”He wished His disciples to imitate the meekness of Moses and the zeal of Elijah.\” St. Hilary of Poitier adds a 6th reason—namely, to signify that He had been foretold by the Law, which Moses gave them, and by the prophets, of whom Elijah was the principal.
Lofty mysteries should not be immediately explained to everyone, but should be handed down through superiors to others in their proper turn. Consequently, as Chrysostom says (on Mt. 17:3), \”He took these three as being superior to the rest.\” For \”Peter excelled in the love\” he bore to Christ and in the power bestowed on him; John in the privilege of Christ\’s love for him on account of his virginity, and, again, on account of his being privileged to be an Evangelist; James on account of the privilege of martyrdom. Nevertheless He did not wish them to tell others what they had seen before His Resurrection; \”lest,\” as Jerome says on Mt. 17:19, \”such a wonderful thing should seem incredible to them; and lest, after hearing of so great glory, they should be scandalized at the Cross\” that followed; or, again, \”lest [the Cross] should be entirely hindered by the people\” [*Bede, Hom. xviii; cf. Catena Aurea]; and \”in order that they might then be witnesses of spiritual things when they should be filled with the Holy Ghost\” [*Hilary, in Matth. xvii].