What then? — Not “What art thou then?” but expressing surprise at the answer, and passing on with impatience to the alternative, “Art thou Elias?” (Comp. on this and the following question, Deuteronomy 18:15; Deuteronomy 18:18; Isaiah 40 ff.; Malachi 4:5; 2MMalachi 2:1-8; and 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, still less “the prophet,” by which, whether thinking of the words of Moses or the fuller vision of Isaiah from which he immediately quotes, he would understand the Messiah himself,
Related Commentaries of John 1:21
John 1:21
21 And they asked him, What then? Art thou Elias? And he saith, I am not. Art thou that prophet? And he answered, No.