9. And as they came down from the mountain, Jesus charged them, saying, Tell the vision to no man, until the Son of man be risen again from the dead.
JEROME: It follows; And as they came down from the mount, Jesus charged them, saying, Tell no man this vision, until the Son of Man shall rise from the dead. He will not be preached among the people, lest the marvel of the thing should seem incredible, and lest the cross following after so great glory should cause offense.
REMIG; Or, because if His majesty should be published among the people, they should hinder the dispensation of His passion, by resistance to the chief Priests; and thus the redemption of the human race should suffer impediment.
HILARY; He enjoins silence respecting what they had seen, for this reason, that when they should be filled with the Holy Spirit, they should then become witnesses of these spiritual deeds.
10. And his disciples asked him, saying, Why then say the Scribes that Elias must first come?
11. And Jesus answered and said to them, Elias truly shall first come, and restore all things.
12. But I say to you, That Elias is come already, and they knew him not, but have done to him whatsoever they would. Likewise shall also the Son of man suffer of them.
13. Then the disciples understood that he spoke to them of John the Baptist.
JEROME; It was a tradition of the Pharisees following the Prophet Malachi, that Elias should come before the coming of the Savior, and bring back the heart of the fathers to the children, and the children to the fathers, and restore all things to their ancient state. The disciples then consider that this transformation which they had seen in the mount was His coming in glory, and therefore it is said, And his disciples asked, him, saying, How then say the Scribes that Elias must first come? As though they had said, If you have already come in glory, how is it that your forerunner appears not yet? And this they say chiefly because they see that Elias is departed again.
CHRYS; The disciples knew not of the coming of Elias out of the Scriptures; but the Scribes made it known to them; and this report was current among the ignorant multitude, as was that concerning Christ. Yet the Scribes did not explain the coming of Christ and of Elias, as they ought to have done. For the Scriptures speak of two comings of Christ; that which has taken place, and that which is yet to be. But the Scribes, blinding the people, spoke to them only of His second coming, and said, If this be the Christ, then should Elias have come before Him. Christ thus resolves the difficulty, He answered and said, Elias truly shall come, and restore all things; but I say to you, that Elias has already come. Think not that here is a contradiction in His speech, if He first say that Elias shall come, and then that he is come. For when He says that Elias shall come and restore all things, He speaks of Elias himself in his own proper person, who indeed shall restore all things, in that he shall correct the unbelief of the Jews, who shall then be to be found; and that is the turning the hearts of the fathers to the children, that is, the hearts of the Jews to the Apostles.
AUG; Or; He shall restore all things, that is those whom the persecution of Antichrist shall have overthrown; as He Himself should restore by His death those whom He ought.
CHRYS; But if there shall so much good arise out of the presence of Elias, why did He not send him at that time? We shall say, Because they then held Christ to be Elias, and yet believed not on Him. But they shall hereafter believe Elias, because when he shall come after so great expectation announcing Jesus, they will more readily receive what shall be taught by Him. But when He says that Elias is come already, He calls John the Baptist Elias from the resemblance of their ministry; for as Elias shall be the forerunner of His second coming, so was John the forerunner of His first. And He calls John Elias, to show that His first coming was agreeable to the Old Testament, and to prophecy.
JEROME; He then who at the Savior's second coming should come in the truth of His body, come now in John in power and spirit. It follows, And they knew him not, but did to him whatsoever they would, that is, despised and beheaded him.
HILARY; As he announced the Lord's coming, so he was also to foreshow His passion by the example of his own suffering and wrong; whence it follows, So also shall the Son of Man suffer of them.
CHRYS; He takes the opportunity from the passion of John to refer to His own passion, thus giving them much comfort.
JEROME; It is inquire how, seeing that Herod and Herodias were they that killed John, it can be said that Jesus also was crucified by them, when we read that He was put to death by the Scribes and Pharisees? It must be answered briefly, that the party of the Pharisees consented to the death of John, and that in the Lord's crucifixion Herod united his approval, when having mocked and set Him at nothing, he sent Him back to Pilate, that he should crucify Him.
RABAN; From the mention of His own passion which the Lord had often foretold to them, and from that of His forerunner, which they beheld already accomplished, the disciples perceived that John was set forth to them under the name of Elias; whence it follows; Then understood the disciples that he spoke to them of John the Baptist.
ORIGEN; That He says of John, Elias is already come, is not to be understood of the soul of Elias, that we fall not into the doctrine of metempsychosis, which is foreign to the truth of Church doctrine, but, as the Angel had foretold, he came in the spirit and power of Elias.
Catena Aurea Matthew 17