John Chrysostom — as excerpted in the Catena Aurea on Luke 9:28-31

“(Hom. 56. in Matt.) Or else this took place because the multitude said He was Elias or Jeremias, to shew the distinction between our Lord and His servants. And to make it plain that He was not an enemy of God, and transgressor of the law, He shewed these two standing by Him; (for else, Moses the lawgiver, and Elias who was zealous for the glory of God, had not stood by Him,) but also to give testimony to the virtues of the men. For each had ofttimes exposed Himself to death in keeping the divine commands. He wishes also His disciples to imitate them in the government of the people, that they might be indeed meek like Moses, and zealous like Elias. He introduces them also to set forth the glory of His cross, to console Peter and the others who feared His Passion. Hence it follows, And spake of his decease, which he should accomplish at Jerusalem.”