John Chrysostom

Not then from the Scriptures did they know this, but the Scribes used to explain themselves, and this saying was reported abroad amongst the ignorant people; as about Christ also. Wherefore the Samaritan woman also said, "Messiah cometh; when He is come, He will tell us all things:" and they themselves asked John, "Art thou Elias, or the Prophet?" For the saying, as I said, prevailed, both that concerning the Christ and that concerning Elias, not however rightly interpreted by them. …

Homily on the Gospel of Matthew 57 ↗

Theophylact of Ohrid

And His disciples asked Him, saying, Why then say the scribes that Elijah must first come? The scribes were deceiving the people, saying that Jesus was not the Christ, for if He were, Elijah would first have come. But they did not know that there are two comings of Christ, the Forerunner of the first being John, and of the second, Elijah. Christ then explains this to the disciples.

Commentary on Matthew ↗

Jerome

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? …

Commentary on Matthew ↗

Thomas Aquinas

And the disciples asked him, etc. In this part he satisfies the question of the disciples. And first the question is presented; secondly, the response, at but he answering, etc.; thirdly, the effect, at then the disciples understood, etc. The apostles, seeing him transformed, believed that from that moment he would begin to reign. For they had understood that Elijah was to come first, Malachi 4:5. …

Commentary on Matthew ↗