Luke 9:31, [Moses and Elijah] ‘were speaking about His exodus (ἔξοδος) which He was about to achieve in Jerusalem.’

Notice that it is AS Jesus speaks with Moses and Elijah that His death (which in 9:22 is presented as something that happens TO Him) is spoken of as the Exodus that He will ACHIEVE. There are a number of significant typological layers to the presence of Moses and Elijah on the mountain with Jesus, but one of the most significant comes as we note that these men represent the Law (Moses) and the Prophets (Elijah); in other words, they represent the totality of the Hebrew Scriptures, the Old Testament (OT).

So, it is when Moses and Elijah—that is, the OT—speak with Jesus that Jesus’ death is revealed to actually be the divine redemption that He accomplishes for His people. Though I believe the transfiguration really did take place in space and time, it is also a potent image for what we read in Luke 24:27, namely, that the OT is fundamentally an exposition of previously-veiled glory of the crucified and risen Jesus…The Law and the Prophets (Moses and Elijah) speak of HIM.

When we realize that the entire OT is about Christ, then, when we hear the Law and Prophets speaking about the works of YHWH, we realize that they are ultimately speaking of JESUS. The anastasiform Son is the sole theme of Scripture (Lk. 9:35-36). As we listen to the OT in this way, Jesus is quite literally transfigured before our eyes…we realize that YHWH blazing in the midst of the thorny bush IS the one suspended on the tree of our curse, that the one who divides the red sea IS the one who passes through the sea of our troubles and abyss of our condemnation on Calvary, that the one who stands upon the stricken stone and gives water to the people IS He who—as the Rock of Ages—is pierced on the cross so that the river of living water might flow into the wilderness of His people’s spiritual death…

In ‘conversation’ with the Law and Prophets, His disciples—you and I—can, even to this day, behold the slain and risen Jesus transfigured into the radiance of the glory of God.