6 For mark, beloved brethren, how true it is what I say. When John was conceived, or rather when he was born, the Holy Spirit prophesied that this would be fulfilled in him: “And he shall be,” he said, “the forerunner of the Highest, in the spirit and power of Elias.” What signifies “in the spirit and power of Elias”? In the same Holy Spirit in the room of Elias. Wherefore in room of Elias? Because what Elias will be to the second, that John was to the first advent.
Rightly therefore, speaking literally, did John reply. For the Lord spoke figuratively, “Elias, the same is John:” but he, as I have said, spoke literally when he said, “I am not Elias.” Neither did John speak falsely, nor did the Lord speak falsely; neither was the word of the herald nor of the Judge false, if only you understand. But who shall understand? He who shall have imitated the lowliness of the herald, and shall have acknowledged the loftiness of the Judge. For nothing was more lowly than the herald.
My brethren, in nothing had John greater merit than in this humility, inasmuch as when he was able to deceive men, and to be thought Christ, and to have been received in the place of Christ (for so great were his grace and his excellency), nevertheless he openly confessed and said, “I am not the Christ.” “Are you Elias?” If he had said I am Elias, it would have been as if Christ were already coming in His second advent to judge, not in His first to be judged. As if saying, Elias is yet to come, “I am not,” said he, “Elias.”
But give heed to the lowly One before whom John came, that you may not feel the lofty One before whom Elias came. For thus also did the Lord complete the saying: “John the Baptist is he which is to come.” He came as a figure of that in which Elias is to come in his own person. Then Elias will in his own proper person be Elias, now in similitude he was John. Now John in his own proper person is John, in similitude Elias. The two heralds gave to each other their similitudes, and kept their own proper persons; but the Judge is one Lord, whether preceded by this herald or by that.
Source: Tractates on the Gospel of John (New Advent)