The Transfiguration event has caused wonder right from the moment that it happened - Peter, James and John certainly were awestruck by it. But why were Elijah and Moses in it? After all, weren't they "Old Testament"? What were they doing in the New Testament?
Of the two, Moses is perhaps the most straight forward. Moses was selected as a representative for The Law of God, summed up by The Ten Commandments. And Jesus said of The Law:
But why Elijah?
As the above verses state, "Think not that I have come to abolish the Law and the prophets; I have come not to abolish them but to fulfil them." Just as The Law of God, the foundation of the future Kingdom Of God, spans the entire Bible, so too does the role of the prophets, to provide a witness and a warning. Elijah, one of the earliest prophets, and perhaps the most zealous, was chosen to represent the function of the prophets.
Since thousands of years of Bible History span more than a single human lifetime, the term "Elijah," continued to be used to represent the prophets (just as "Washington" has come to sometimes mean successive American government administrations - keeping in mind that the city was named after a man named Washington who has himself long ago died).
And then came John The Baptist, the "Elijah" of his time:
That same "spirit of Elijah," which is actually the Holy Spirit of God in the prophet, will again surface, in the most powerful and spectacular version ever, in The Two Witnesses just prior to The Return Of Jesus Christ:
"And I will grant my two witnesses power to prophesy for one thousand two hundred and sixty days, clothed in sackcloth." These are the two olive trees and the two lampstands which stand before the Lord of the earth. And if any one would harm them, fire pours out from their mouth and consumes their foes; if any one would harm them, thus he is doomed to be killed. They have power to shut the sky, that no rain may fall during the days of their prophesying, and they have power over the waters to turn them into blood, and to smite the earth with every plague, as often as they desire." (Revelation 11:3-6 RSV)
Fact Finder: What was the purpose of God's prophets?