Commentaries:
When John the Baptist replied, he knew in advance what they were thinking because he knew what the Jews believed in regard to Elijah. This is why he answered, "I am not Elijah." In other words, since he was preaching and doing certain things, they expected that he was Elijah. The definite article is left out: "No. I am not Elijah."
The reason he answers this way is because he probably did not know at this time that he was the Elijah of Malachi 4:5, so he answered honestly the only way he could: "No, I am not the resurrected Elijah."
John W. Ritenbaugh
Prophets and Prophecy (Part Three)
Other Forerunner Commentary entries containing John 1:21:
Luke 1:17