(Verse 8.) But lifting up their eyes, they saw no one except Jesus alone. Reasonably, after they had risen, they saw no one except Jesus alone: lest if Moses and Elijah had continued with the Lord, the uncertain voice of the Father would be seen, to whom he would give the testimony. Therefore, they see Jesus standing, the cloud having been taken away, and Moses and Elijah had vanished: because after the shadow of the Law and the Prophets had departed, which had covered the apostles with its veil, both lights are found in the Gospel.
17:9
(Verse 9) And as they were coming down the mountain, Jesus commanded them, saying: Tell no one the vision, until the Son of Man rises from the dead. The anticipation of the coming kingdom, and the glory of the triumphant one, had been shown on the mountain. Therefore, He does not want this to be preached to the people, lest it be regarded as unbelievable for the greatness of the matter, and lest the cross, following such glory, would become a stumbling block to simple minds.
17:10
(Verse 10.) And the disciples asked him, saying, Why then do the scribes say that Elijah must come first? Unless we know the reasons why the disciples asked about the name of Elijah, their questioning seems foolish and extraordinary. For what does it matter to inquire about the coming of Elijah, which is written above? The tradition of the Pharisees is, according to the prophet Malachi (who is the last of the twelve), that Elijah will come before the coming of the Savior and restore the hearts of the fathers to the children, and the hearts of the children to the fathers, and restore everything to its former state (Malachi 4). Therefore, the disciples think that this transformation of glory, which they see on the mountain, is the one they have seen, and they say: If you have already come in glory, why does your precursor not appear, especially since they saw Elijah depart? However, when they add: The scribes say that Elijah must come first; by saying 'first,' they show that unless Elijah comes, the coming of the Savior according to the Scriptures is not fulfilled.
17:11-12
(Verse 11, 12.) But he answered and said unto them, Elias indeed cometh first, and restoreth all things; and how it is written of the Son of man, that he must suffer many things, and be set at nought. But I say unto you, That Elias is indeed come, and they have done unto him whatsoever they listed, as it is written of him.
And they did not recognize him; but they did to him whatever they wanted: That is, they despised him, and beheaded him.
17:13
(Verse 13) And so the Son of Man will suffer at their hands. Then the disciples understood that he was speaking to them about John the Baptist. Now, as Herod and Herodias are said to have killed John, how can they also say that they crucified Jesus, when we read that he was killed by the scribes and Pharisees? The answer is that both the Pharisees were in agreement with the death of John, and Herod willingly participated in the killing of the Lord. He sent him, mocked and despised, to Pilate to be crucified.
17:14-15
(Vers. 14, 15.) Lord, have mercy on my son, for he is a lunatic and suffers greatly: for he often falls into fire and frequently into water. And I brought him to your disciples, but they could not heal him. This is why the devil, observing the course of the moon, seizes upon men and strives to defame the Creator through His creatures, as we have said above. But it seems to me, according to the spiritual interpretation, that the lunatic is one who is constantly changing to vice in the moments of the hours, not persisting in what he has begun, but growing and diminishing: and now he is carried to the fire, by which the hearts of adulterers are inflamed (Hosea VII): now to the waters, which cannot quench charity. But when he says, 'I brought him to your disciples, and they could not heal him,' he secretly accuses the apostles, since the inability to heal sometimes refers not to the weakness of those who are healing, but to the faith of those who are being healed, as the Lord says, 'Let it be done to you according to your faith' (Mark 5:34, and 10:52).
17:16
(Verse 16.) But Jesus replied, saying: O unbelieving and perverse generation, how long shall I be with you? How long shall I bear with you? Bring him here to me. Not because he was overcome by weariness, and gentle and meek, who did not open his mouth like a lamb before the shearer (Isaiah 53), nor did he burst forth in words of fury; but because, in the likeness of a physician, if he sees a sick person behaving contrary to his instructions, he may say: How long shall I come to your house, until I lose the skill of my craft, you commanding one thing and doing another? But he is not angry with the man, but with the fault; and through one man he accuses the Jews of unfaithfulness, so that he immediately says: Bring him here to me.
17:17
(Verse 17) And Jesus rebuked him, and the demon came out of him, and the boy was healed from that hour. It was not the boy who suffered, but rather the demon who ought to be rebuked. Whether he rebuked the boy, and the demon came out of him: because he had been oppressed by the demon due to his own sins.
17:18
(Verse 18) And they said, why could we not cast him out? Jesus said to them: Because of your unbelief. Amen I say to you. This is what he says in another place: Whatever you ask in my name, believing, you will receive (John 15:2). Therefore, whenever we do not receive, it is not due to the impossibility of the one granting, but the fault of those praying.
17:19
(Verse 19.) If you have faith as a grain of mustard seed, you shall say to this mountain: Remove from hence, and it shall remove: and nothing shall be impossible to you. Some people think that faith compared to a grain of mustard seed is small, because the kingdom of heaven is compared to a grain of mustard seed; when the apostle says: And if I shall have all faith, so that I could remove mountains (I Cor. XV, 2). Therefore, great is the faith that is compared to a grain of mustard seed. The translation of the mountain does not signify the one that we see with our physical eyes, but rather the one who has been transferred by the Lord from the lunatic. For when he says, 'You will say to this mountain, "Move from here to there," and it will move,' it is understood to refer to the devil. From this, those are shown to be foolish who argue that the apostles and all believers, did not even have a little faith, because none of them moved mountains. For the movement of a mountain from one place to another is not beneficial in itself, and seeking the vain display of miracles; but this mountain must be moved for the benefit of all, which is said by the prophet to corrupt all the earth (Zech. IV).
17:20
(Verse 20.) However, this kind does not go out, except by prayer and fasting. While he teaches how the most wicked demon can be expelled, he instructs everyone for life.
17:21-23
(Ver. 21 seq.) While they were conversing in Galilee, Jesus said to them: The Son of Man is to be handed over to the hands of men, and they will kill him; and on the third day he will rise again. And they were greatly saddened. He always mixes sorrows with prosperity, so that when they come suddenly, they do not terrify the apostles; but they are carried by their prepared minds: For if they are saddened by his impending death, they should be joyful at the news of his resurrection on the third day. Furthermore, their grief and sorrow are not due to disbelief (otherwise they would have known that Peter was rebuked because he did not understand the things of God, but those of men), but rather because out of love for their master, they are willing to hear nothing negative or humiliating about him.
17:24