John Gill

Then the disciples understood,.... 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.