Juan 1:21

The leaders asked John if he was Elijah because messianic expectation was high then due to Daniel’s prediction that dated the appearance of Messiah then (Daniel 9:25). Malachi had predicted that Elijah would return to herald the day of the Lord that Messiah would inaugurate (Malachi 4:5-6).

"Popularly it was believed that Elijah would anoint the Messiah, and thereby reveal his identity to him and to Israel (see Justin, Apology 35.1)." [Note: Beasley-Murray, p. 24.]

When John the Baptist denied being Elijah, he was denying being Elijah himself. His dress, diet, lifestyle, and ministry, however, were very similar to Elijah’s.

The prophet whom the leaders had in mind when they asked their third question was the prophet that Moses had predicted would come (Deuteronomy 18:15-18). Merrill pointed out that of the 42 New Testament citations of Deuteronomy 18:15-19, 24 of them appear in John’s Gospel. [Note: Eugene H. Merrill, "Deuteronomy, New Testament Faith, and the Christian Life," in Integrity of Heart, Skillfulness of Hands, p. 27.] This prophet would bring new revelation from God and might lead the Israelites in a new Exodus and overcome their oppressors. The Jews incorrectly failed to identify this prophet with Messiah (cf. John 7:40-41). In contrast, the earliest Christian preachers contended that "the prophet" was identical with the Messiah (cf. Acts 3:22). John the Baptist claimed that he was not that long-expected prophet any more than he was the Messiah or Elijah.