Matthew 17:23

Thus does He ever mix the joyful and the grievous; if it grieves them that He is to be put to death, they ought to be gladdened when they hear, And shall rise again the third day. That they were thus made exceeding sorrowful, came not of their lack of faith; but out of their love of their Master they could not endure to hear of any hurt or indignity for Him.

That is, to hinder their saying, "wherefore do we abide here continually," He speaks to them again of the passion; on hearing which they had no wish so much as to see Jerusalem. And it is remarkable how, when both Peter had been rebuked, and Moses and Elias had discoursed concerning it, and had called the thing glory, and the Father had uttered a voice from above, and so many miracles had been done, and the resurrection was at the doors (for He said, He should by no means abide any long time in death, but should be raised the third day); not even so did they endure it, but were sorry; and not merely sorry, but exceeding sorry. Now this arose from their being ignorant as yet of the force of His sayings. This Mark and Luke indirectly expressing said, the one, "They understood not the saying, and were afraid to ask Him:" the other, "It was hid from them, that they perceived it not, and they feared to ask Him of that saying." And yet if they were ignorant, how were they sorry? Because they were not altogether ignorant; that He was to die they knew, continually hearing it, but what this death might be, and that there would be a speedy release from it, and that it would work innumerable blessings, as yet they knew not clearly; nor what this resurrection might be: but they understood it not, wherefore they grieved; for indeed they clung very earnestly to their Master.

"Likewise shall also the Son of Man suffer of them." Seest thou how again He in due season reminds them of His passion, laying up for them great store of comfort from the passion of John. And not in this way only, but also by presently working great miracles. Yea, and whensoever He speaks of His passion, presently He works miracles, both after those sayings and before them; and in many places one may find Him to have kept this rule. "Then," for instance, it saith, "He began to signify how that He must go unto Jerusalem, and be killed, and suffer many things." "Then:" when? when He was confessed to be Christ, and the Son of God. Again on the mountain, when He had shown them the marvellous vision, and the prophets had been discoursing of His glory, He reminded them of His passion. For having spoken of the history concerning John, He added, "Likewise shall also the Son of Man suffer of them." And after a little while again, when He had cast out the devil, which His disciples were not able to cast out; for then too, "As they abode in Galilee," so it saith, "Jesus said unto them, The Son of Man shall be betrayed into the hands of sinful men, and they shall kill Him, and the third day He shall rise again." Now in doing this, He by the greatness of the miracles was abating the excess of their sorrow, and in every way consoling them; even as here also, by the mention of John's death, He afforded them much consolation. But when they heard these things, they do not ask Him when Elias cometh; being straitened either by grief at His passion, or by fear. For on many occasions, upon seeing Him unwilling to speak a thing clearly, they are silent, and so an end. […]

Therefore he brought the disciples to the mountain and showed them the glory with which he will shine on the universe in the future. Then coming down from the mountain he freed a person from a rough and evil spirit. It was altogether necessary for him to undergo his saving Passion for us and to suffer the violence of the Jews. When this happened, it was quite likely that the disciples would be alarmed and would ponder it and say among themselves: "He has raised so many from the dead by divine power, he commands the seas and winds, he overwhelms Satan with his words—how has he now been taken and fallen to the noose of his murderers? Then maybe we were deceived when we thought that he was God?" So that they should know the future fully and completely, therefore, he foretells to them the mystery of the Passion.

Or, fasting is here understood generally as abstinence not from food only, but from all carnal allurements, and sinful passions. In like manner prayer is to be understood in general as consisting in pious and good acts, concerning which the Apostle speaks, Pray without ceasing. (1 Thess. 5:17.) The Lord often foretold to His disciples the mysteries of His passion, in order that when they come to pass, they might be the lighter to them from having been known beforehand.