Charles Ellicott Commentary John 1:21

"And they asked him, What then? Art thou Elijah? And he saith, I am not. Art thou the prophet? And he answered, No." — John 1:21 (ASV)

What then?—This was not "What art thou then?" but rather an expression of surprise at the answer, followed by an impatient move to the alternative question: "Art thou Elias?" (Compare on this and the following question, Deuteronomy 18:15; Deuteronomy 18:18; Isaiah 40 and following; Malachi 4:5; and the note on Matthew 16:14).

The angel had announced that he shall go before Him in the spirit and power of Elias. The Lord declared, Elias is come already (Matthew 18:12–13). And yet the Forerunner can assert that—in the literal sense in which they ask the question and would understand the answer—he is not Elias, and still less "the prophet"—a title by which he (John the Baptist) would understand the Messiah himself, whether he was thinking of the words of Moses or of the fuller vision of Isaiah, from which he immediately quotes.