John 1:21 "And they asked him (John the Baptist)…Are you Elias? And he said, I am not." John the Baptist was not in a literal way Elijah returned in body form, which some apparently thought. So, why did Jesus later say that John the Baptist was Elijah? "And if you are willing to receive and accept it, John himself is Elijah who was come," Matthew 11:13-14, Amplified Bible. Jesus was telling us that Elijah was a figure or symbol of John the Baptist - that Elijah represented John the Baptist. Therefore we should try to learn how the lives and work of both men correspond.
The angel Gabriel compared the work of Elijah and John the Baptist. "And he (John the Baptist) shall go before him (Jesus) in the spirit and power of Elijah…to make ready a people prepared for the Lord (Jesus)," Luke 1:17. Elijah worked to reform the hearts of the Jewish people so that they would return to worship the true God, Jehovah. The angel Gabriel announced that John the Baptist would also work to reform the hearts of the Jews so that they would accept Jesus as Messiah. John the Baptist was the forerunner of Jesus, and he was to prepare the people to accept Jesus at his first advent.
Did either man fully succeed in his mission? Neither one reformed the entire Jewish nation, because men's hearts were not ready. Jesus understood this when he said, "And if you are willing to receive and accept it, (John's announcement that Jesus is the Messiah)," Matthew 11:14. In this way, John's reform work was a fulfillment of Elijah's reform work. He preached about Jesus but most of the people were not ready to receive him and to follow him at his first advent.
If John the Baptist had a work which was not completely successful at Jesus' first advent, is there a corresponding less-than-successful work at Jesus' second advent, when he returns to earth? This is exactly what the prophet Malachi says in Malachi 4:5 (Amplified Bible): "Behold I send you Elijah the prophet before the great and terrible day of the Lord comes." This "day of the Lord" is just before the establishment of Christ's righteous kingdom on earth. The trouble occurs because men are not prepared to receive the message of the Lord's return or of the Lord's actual return despite the Elijah/John the Baptist work of announcing his second advent and the setting up of Christ's kingdom.
Beyond that day is the most wonderful prophecy of Christ's kingdom on earth told by all God's holy prophets. "And he shall send Jesus Christ, which before was preached unto you: Whom the heaven must receive until the times of restitution of all things, which God hath spoken by the mouth of all his holy prophets since the world began," Acts 3:19-21. True and complete restitution and restoring of men's hearts to follow the Lord will eventually occur because the Christ will be reigning in his righteous kingdom. He will accomplish that great work that was only foreshadowed by the work of Elijah and John the Baptist. We joyfully await this glorious prospect.