Theophylact of Ohrid

And Jesus answered and said unto them, Elijah truly shall first come, and restore all things. By saying, "Elijah truly shall come," Christ shows that he has not yet come, but that he will appear as the forerunner of the second coming; and when he appears, he will restore all teachable Jews to faith in Christ, as if restoring them to their paternal inheritance which they had lost.

Commentary on Matthew ↗

Augustine of Hippo

Or; He shall restore all things, that is those whom the persecution of Antichrist shall have overthrown; as He Himself should restore by His death those whom He ought.

Catena Aurea by Aquinas ↗

Jerome

But he answered and said unto them, Elias indeed cometh first, and restoreth all things; and how it is written of the Son of man, that he must suffer many things, and be set at nought. But I say unto you, That Elias is indeed come, and they have done unto him whatsoever they listed, as it is written of him. And they did not recognize him; but they did to him whatever they wanted: That is, they despised him, and beheaded him.

Commentary on Matthew ↗

Thomas Aquinas

The Lord satisfies this doubt. And first he touches upon the future coming; secondly, the past. Hence he says but he answering said to them: Elijah indeed shall come. Hence he speaks of a twofold Elijah, namely, of Elijah in his own person who is to come: and he shall come to announce the way of justice, and shall restore all things, and shall turn the hearts of men to Christ, and shall convert the Jews to the faith of the patriarchs who had faith in Christ, because, as is found in Romans 11:25, blindness in part has happened in …

Commentary on Matthew ↗