The disciples knew not of the coming of Elias out of the Scriptures; but the Scribes made it known to them; and this report was current among the ignorant multitude, as was that concerning Christ. Yet the Scribes did not explain the coming of Christ and of Elias, as they ought to have done. For the Scriptures speak of two comings of Christ; that which has taken place, and that which is yet to be. But the Scribes, blinding the people, spake to them only of His second coming, and said, If this be the Christ, then should Elias have come before Him. Christ thus resolves the difficulty, He answered and said, “Elias truly shall come, and restore all things; but I say unto you, that Elias has already come.”
Think not that here is a contradiction in His speech, if He first say that Elias shall come, and then that he is come. For when He says that Elias shall come and restore all things, He speaks of Elias himself in his own proper person, who indeed shall restore all things, in that he shall correct the unbelief of the Jews, who shall then be to be found; and that is the turning the hearts of the fathers to the children, that is, the hearts of the Jews to the Apostles
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: Hom., lvii