John Chrysostom — as excerpted in the Catena Aurea on Matthew 17:1-4

“There are many reasons why these should appear. The first it, this; because the multitudes said He was Elias, or Jeremias, or one of the Prophets, He here brings with Him the chief of the Prophets, that hence at least may be seen the difference between the servants and their Lord. Another reason is this; because the Jews were ever charging Jesus with being a transgressor of the Law and blasphemer, and usurping to Himself the glory of the Father, that He might prove Himself guiltless of both charges, He brings forward those who were eminent in both particulars; Moses, who gave the Law, and Elias, who was jealous for the glory of God. Another reason is, that they might learn that He has the power of life and death; by producing Moses, who was dead, and Elias, who had not yet experienced death. A further reason also the Evangelist discovers, that He might shew the glory of His cross, and thus soothe Peter, and the other disciples, who were fearing His death; for they talked, as another Evangelist declares, of His decease which He should accomplish at Jerusalem. Wherefore He brings forward those who had exposed themselves to death for God’s pleasure, and for the people that believed; for both had willingly stood before tyrants, Moses before Pharaoh, Elias before Ahab. Lastly, also, He brings them forward, that the disciples should emulate their privileges, and be meek as Moses, and zealous as Elias.”