5 They saw Him then lowly, and did not know Him. He was pointed out to them by a lamp. For in the first place he, than whom no greater had arisen of those born of women, said, “I am not the Christ.” It was said to him, “Are you Elias? He answered, I am not.” For Christ sends Elias before Him: and he said, “I am not,” and occasioned a question for us. For it is to be feared lest men, insufficiently understanding, think that John contradicted what Christ said. For in a certain place, when the Lord Jesus Christ said certain things in the Gospel regarding Himself, His disciples answered Him: “How then say the scribes,” that is, those skilled in the law, “that Elias must first come?”
And the Lord said, “Elias is already come, and they have done unto him what they listed;” and, if you wish to know, John the Baptist is he. The Lord Jesus Christ said, “Elias is already come, and John the Baptist” is he; but John, being interrogated, confessed that he was not Elias, in the same manner that he confessed that he was not Christ. And as his confession that he was not Christ was true, so was his confession that he was not Elias. How then shall we compare the words of the herald with the words of the Judge?
Away with the thought that the herald speaks falsehood; for that which he speaks he hears from the Judge. Wherefore then did he say, “I am not Elias;” and the Lord, “He is Elias”? Because the Lord Jesus Christ wished in him to prefigure His own advent, and to say that John was in the spirit of Elias. And what John was to the first advent, that will Elias be to the second advent. As there are two advents of the Judge, so are there two heralds. The Judge indeed was the same, but the heralds two, but not two judges. It was needful that in the first instance the Judge should come to be judged. He sent before Him His first herald; He called him Elias, because Elias will be in the second advent what John was in the first.
Source: Tractates on the Gospel of John (New Advent)