“He will go before the Lord in the spirit and power of Elijah” (Lk 1:17)
“Why do the scribes (that is to say, the Doctors of the Law) say that Elijah has to come first?” The Lord answers them: “Elijah has already come and they did not recognize him but did to him whatever they pleased, and if you wish to understand, he is John the Baptist.” In this way our Lord Jesus Christ expressly said: “Elijah has already come” and that this refers to John the Baptist. However, when John was questioned, he admitted that he was no more Elijah than he was the Messiah (Jn 1:20f.)... Now why did he admit: “I am not Elijah” whereas the Lord said to his disciples that he was Elijah? Our Lord wanted to speak in symbols of his future coming and say that John had come in the spirit of Elijah. What John was for the first coming, Elijah will be for the second. There are two comings for the Judge; there are also two precursors. The Judge is the same in both comings but there are two precursors... The Judge must come first of all to be judged. But he sent a first precursor before him and called him Elijah because Elijah will become for the second coming what John was for the first.
Consider, beloved brethren, how well this explanation is founded on truth. When John was conceived... the Holy Spirit made this prophecy which would be fulfilled in him: “He will go before the Most High in the spirit and power of Elijah” (Lk 1:17)... Who can understand these things? The person who has imitated the precursor's humility and has known the majesty of the Judge. No one has been humbler than this holy precursor. This humility of John's constitutes his greatest merit. So great were his grace and virtue that he could have deceived people, passed himself off as the Messiah, been thought of as the Messiah. And yet he openly declared: "I am not the Messiah. - Are you Elijah?... - I am not Elijah."