And there appeared unto them Elijah with Moses. Moses and Elijah were there because Moses was the lawgiver of the old covenant, and Elijah was conspicuous among the prophets; so that they were the representatives, the one of the Law, and the other of the "goodly fellowship of the prophets.
They appear together to bear witness to Christ as the true Messiah, the Savior of the world, prefigured in the Law, and foretold by the prophets. They appear to bear witness to him, and then to resign their offices to the great Lawgiver and Prophet whom they foreshadowed.
Then, further, Moses died, but Elijah was translated. Moses, therefore, represents the dead saints who shall rise from their graves and come forth at his coming, while Elijah represents those who shall be found alive at his advent.
Our Lord brought with him, at his transfiguration, Moses who had died, and Elijah who had been translated, that he might show his power over both "the quick and the dead." St. Luke 9:31 says that Moses and Elijah "appeared in glory, and spake of his decease ( τὴν ἔξοδον αὐτοῦ) which he should accomplish at Jerusalem."
They appeared in glory; the Divine splendor irradiated them. They "spake of his decease," literally, his departure—his departure not only out of Jerusalem, but out of this life, by his death upon the cross.
The death of Christ was thus shown to be the ultimate end to which the Law and the prophets pointed. Even in that hour of his glory, on the Mount of Transfiguration, this was their theme; and thus the disciples were nerved to look with hope and faith to that which they had contemplated with dismay.