Luke says of the transfiguration that Elijah and Moses “appeared in glory and spoke of His decease which He was about to accomplish at Jerusalem” (Lk. 9:31).
Literally, Luke says that they talked of His “exodus” which He was about to “fulfill” in Jerusalem.
As Matthew’s use of the word “fulfill” also indicates, Jesus means to exemplify, embody, incarnate the words of Moses and the Prophets. He is the Word made flesh, and therefore He becomes the words of the law and the prophets. He is the archetype toward which all of the words of the Old Testament pointed. The Old Covenant words were all types and shadows of the Christ. They were signs of God coming in the flesh.
And Luke’s summary of the conversation between Jesus, Moses, and Elijah, emphasizes this point. Jesus came to fulfill the exodus. He came to do what the original exodus pointed toward. He came to “fill up” and “fill out” all that the original exodus and all the other mini-exoduses only previewed and pointed to.
We say that Jesus is the “greater Moses” and the “greater Elijah,” but literally this means that their words and actions are taken up into His person. He is all of the law and the prophets in the flesh, in person. This is because the Spirit that filled and inspired the OT writers and actors was the Spirit of Christ (1 Pet. 1:10-12).
This establishes the authority and authenticity of the Scriptures, but it also pictures what the Spirit does in every believer. Apart from the Spirit, the Scriptures are words on a page and we are dry bones in a valley. But the gift of the Spirit creates, incarnates, gives and makes flesh.
In other words, the Old Covenant was like the body of Jesus scattered by the Spirit through time in words and types and symbols, and in the New Covenant it has been bound up together by the Spirit, in the Word made flesh, born of Mary. Then at Pentecost, the crucified and risen Christ, triumphant over sin and death and Satan, has poured out His victorious Spirit on the Church in order to incarnate that life, that story in us.