John Chrysostom
But what Christ answered to this, is seen by what follows, And he answered and told them, Elias verily cometh first, and restoreth all things; in which He shews that Elias will come before His second advent. For the Scriptures declare two advents of Christ, namely, one which has taken place, and another which is to come; but the Lord asserts that Elias is the forerunner of the second advent.
Where He not only orders them to be silent, but mentioning His Passion, He implies the cause why they were to be silent.
The design of the disciples in asking this question seems to me to be this. We indeed have seen Elias with Thee, and have seen Thee before seeing Elias, but the Scribes say that Elias cometh first; we therefore believe that they have lied.
Now as the Lord asserted that Elias was to be the forerunner of the second advent, so consequently He asserted that John was the forerunner of the first; wherefore He subjoins, But I say unto you, that Elias is indeed come.
But they, being ignorant of the mystery of the resurrection, took hold of that saying, and disputed one with another; wherefore there follows, And they kept that saying with themselves, questioning one with another what the rising from the dead should, mean.
Or else, the disciples asked Jesus, how it was written that the Son of man must suffer? Now in answer to this, He says, As John came in the likeness of Elias, and they evil intreated him, so according to the Scriptures must the Son of man suffer.