Matthew 17:13

“Then the disciples understood, etc.] By his saying that Elias was come, and by the account he gave of his ill usage, it was clear to them, that he spake unto them of John the Baptist; and that he was the Elias that was to come, and was come: so that this observation, that according to prophecy Elias was to come before the Messiah, was no objection to Jesus being the Messiah; but on the contrary, since he that was intended by Elias was come, and had done his work and office, it was a confirmation of the truth of his Messiahship.”

“See also Mark 9:11-13. Why then say the scribes ... - The disciples appear to have been satisfied now that he was the Messiah. The transfiguration had taken away all their doubts, but they recollected that it was a common doctrine among the Jews that Elijah would appear before the Messiah came, and they did not then recollect that he had appeared. To this difficulty the word then refers. “We are satisfied that thou art the Christ, but Elijah has not yet come, as was expected; what, then, is the meaning of the common opinions of our learned men, the scribes? Were they right or wrong in their expectation of Elijah?” See the notes at Matthew 11:14. Matthew 17:11 Elias truly shall first come,…”

“the disciples: Mat 11:14 Matthew 17:14”