From: Mark 9:2-13
 And after six days Jesus took with Him Peter and James and John, and led them up a high mountain apart by themselves; and He was transfigured before them,  and His garments became glistening, intensely white, as no fuller on earth bleach them.  And there appeared to them Elijah with Moses; and they were talking to Jesus.  And Peter said to Jesus, "Master, it is well that we are here; let us make three booths, one for You and one for Moses and one for Elijah."  For he did not know what to say, for they were exceedingly afraid.  And a cloud overshadowed them, and a voice came out of the cloud, "This is My beloved Son; listen to Him."  And suddenly looking around they no longer saw any one with them but Jesus only.  And as they were coming down the mountain, He charged them to tell no one what they had seen, until the Son of Man should have risen from the dead.  So they kept the matter to themselves, questioning what the rising from the dead meant.  And they asked Him, "Why do the scribes say that first Elijah must come?"  And He said to them, "Elijah does come first to restore all things; and how is it written of the Son of Man, that He should suffer many things and be treated with contempt?  But I tell you that Elijah has come, and they did to him whatever they pleased, as it is written of him."
2-10. We contemplate in awe this manifestation of the glory of the Son of God to three of His disciples. Ever since the Incarnation, the divinity of our Lord has usually been hidden behind His humanity. But Christ wishes to show, to these favorite disciples, who will later be pillars of the Church, the splendor of His divine glory, in order to encourage them to follow the difficult way that lies ahead, fixing their gaze on the happy goal which is awaiting them at the end. This is why, as St. Thomas comments (cf. "Summa Theologia", III, q. 45, a. 1), it was appropriate for Him to give them an insight into His glory. The fact that the Transfiguration comes immediately after the first announcement of His passion, and His prophetic words about how His followers would also have to carry His cross, shows us that "through many tribulations we must enter the kingdom of God" (Acts 14:22).
What happened at the Transfiguration? To understand this miraculous event in Christ's life, we must remember that in order to redeem us by His passion and death our Lord freely renounced divine glory and became man, assuming flesh which was capable of suffering and which was not glorious, becoming like us in every way except sin (cf. Hebrew 4:15). In the Transfiguration, Jesus Christ willed that the glory which was His as God and which His soul had from the moment of the Incarnation, should miraculously become present in His body. "We should learn from Jesus' attitude in these trials. During His life on earth He did not even want the glory that belong to Him. Though He had the right to be treated as God, He took the form of a servant, a slave (cf. Philippians 2:6)" ([St] J. Escriva, "Christ Is Passing By", 62). Bearing in mind WHO became man (the divinity of the person and the glory of His soul), it was appropriate for His body to be glorious; given the PURPOSE of His Incarnation, it was not appropriate, usually, for His glory to be evident. Christ shows His glory in the Transfiguration in order to move us to desire the divine glory which will be given us so that, having this hope, we too can understand "that the sufferings of this present time are not worth comparing with the glory that is to be revealed to us" (Romans 8:18).
2. According to Deuteronomy (19:15), to bear witness to anything the evidence of two or three must concur. Perhaps this is why Jesus wanted three Apostles to be present. It should be pointed out that these three Apostles were specially loved by Him; they were with Him also at the raising of the daughter of Jairus (Mark 5:37) and will also be closest to Him during His agony at Gethsemane (Mark 14:33). Cf. note on Matthew 17:1-13.
7. This is how St. Thomas Aquinas explains the meaning of the Transfiguration: "Just as in Baptism, where the mystery of the first regeneration was proclaimed, the operation of the whole Trinity was made manifest, because the Son Incarnate was there, the Holy Spirit appeared under the form of a dove, and the Father made Himself known in the voice; so also in the Transfiguration, which is the sign of the second regeneration [the Resurrection], the whole Trinity appears--the Father in the voice, the Son in the man, the Holy Spirit in the bright cloud; for just as in Baptism He confers innocence, as signified by the simplicity of the dove, so in the Resurrection will He give His elect the clarity of glory and the refreshment from every form of evil, as signified by the bright cloud" ("Summa Theologiae", III, q. 45, 1.4 ad 2). For, really, the Transfiguration was in some way an anticipation not only of Christ's glorification but also of ours. As St. Paul says, "it is the same Spirit Himself bearing witness with our spirit that we are children of God, and if children, then heirs, heirs of God and fellow heirs with Christ, provided we suffer with Him in order that we may also be glorified with Him" (Romans 8:16-17).
10. That the dead would rise was already revealed in the Old Testament (cf. Daniel 12:2-3; 2 Maccabees 7:9; 12:43) and was believed by pious Jews (cf. John 11:23-25). However, they were unable to understand the profound truth of the death and Resurrection of the Lord: they expected a glorious and triumphant Messiah, despite the prophecy that He would suffer and die (cf. Isaiah 53). Hence the Apostles' oblique approach; they too do not dare to directly question our Lord about His Resurrection.
11-13. The scribes and Pharisees interpret the messianic prophecy in Malachi (3:1-2) as meaning that Elijah will appear in person, dramatically, to be followed by the all-triumphant Messiah, with no shadow of pain or humiliation. Jesus tells them that Elijah has indeed come, in the person of John the Baptist (Matthew 17:13) and has prepared the way of the Messiah, a way of pain and suffering.
Verse 12 is a question which Jesus puts to His disciples, but they should really have asked it themselves, had they realized that Christ's Resurrection presupposed the Messiah's suffering and death. Since they fail to ask it, Jesus does, to teach them that He like Elijah (that is, John the Baptist) must experience suffering before entering His glory.
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.