John Chrysostom

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.

Catena Aurea by Aquinas ↗

Theophylact of Ohrid

Among the Jews there circulated a report that before the coming of Christ, Elijah would come. However, the Pharisees did not interpret what was written about Elijah as they should have, but maliciously distorted the meaning of the Scriptures, concealing the truth. For there are two comings of Christ: one, the first (which has already taken place), and another that is yet to be. The forerunner of the first was John; the forerunner of the second will be Elijah. …

Commentary on Mark ↗

Bede

And they asked him, saying: Why then do the Pharisees and Scribes say that Elijah must come first? It is a tradition of the Pharisees, according to Malachi the prophet, who is the last of the twelve, that Elijah will come before the advent of the Savior and will turn the hearts of the fathers to their children, and the hearts of the children to their fathers, and restore all things to their former state. …

On the Gospel of Mark ↗