Theophylact of Ohrid

But I say unto you, That Elijah is come already, and they knew him not, but have done unto him whatsoever they desired. Likewise shall also the Son of Man suffer at their hands. But when Christ says, "Elijah is come already," He is speaking of John the Forerunner; for the Jews "did unto him whatsoever they desired" when they slew him; they slew him when they permitted Herod to slay John, though they could have prevented it.

Commentary on Matthew ↗

Gregory the Dialogist

But when the statement of our Redeemer from another reading is brought to mind, a very complex question arises for us from the words of this reading. For in another place, when asked by his disciples about the coming of Elijah, the Lord replied: "Elijah has already come, and they did not recognize him, but did to him whatever they wished. And if you wish to know, John himself is Elijah." But when John was asked, he says: "I am not Elijah." What is this, dearest brothers, that what Truth affirms, the prophet of Truth denies? …

Forty Gospel Homilies, Homily 7 ↗

Hilary of Poitiers

As he announced the Lord's coming, so he was also to foreshew His passion by the example of his own suffering and wrong? whence it follows, So also shall the Son of Man suffer of them.

Catena Aurea by Aquinas ↗

Thomas Aquinas

Then is added concerning the other Elijah: but I say to you that Elijah has already come. Who is he? John the Baptist, not that he is the same in person, as is found in John 1:21, when it was asked of him, are you Elijah? He answered: no. But in spirit and power: because just as Elijah will be the precursor of the second coming, so John was of the first coming. Likewise, just as Elijah opposed Jezebel, so John opposed Herodias; and just as Elijah was a dweller of the desert, so was John. …

Commentary on Matthew ↗