And besides, Moses and Elijah stood at Jesus' side, and spake with one another of His departure, which He was about, it says, to accomplish at Jerusalem: by which is meant the mystery of the dispensation in the flesh; and of His precious suffering upon the cross. For it is also true that the law of Moses, and the word of the holy prophets, foreshewed the mystery of Christ: the one by types and shadows, painting it, so to speak, as in a picture; while the rest in manifold ways declared beforehand, both that in due time He would appear in our likeness, and for the salvation and life of us all, consent to suffer death upon the tree. The standing, therefore, of Moses and Elijah before Him, and their talking with one another, was a sort of representation, excellently displaying our Lord Jesus Christ, as having the law and the prophets for His body guard, as being the Lord of the law and the prophets, and as foreshown in them by those things which in mutual agreement they before proclaimed. For the words of the prophets are not at variance with the teachings of the law. And this I imagine was what Moses the most priestly and Elijah the most distinguished of the prophets were talking of with one another.
Cyril of Alexandria, Commentary on Luke, Sermon 51.