We will answer this thought-provoking question by explaining the Scriptures that deal directly with your question.
First, John 1:21 "And they asked him…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, what did Jesus mean when he said 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 tells how the work of Elijah and John the Baptist are similar. Elijah had a work of reforming 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 have a work of reforming the hearts of men so that they would accept the one he would introduce: Jesus as Messiah: "And he [speaking to the mother of John the Baptist about her son] 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. 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 his mission? Neither one completed his work for the entire Jewish nation they were sent to reform because men's hearts were not ready. Jesus understood this when he said, "And if you are willing to receive and accept it,[that I am Messiah]," which John the Baptist was announcing, Matthew 11:14. Only those who were willing and had the right heart became Jesus' disciples, but in this way, John's reform work was a fulfillment of Elijah's reform work. John the Baptist 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? We trust that this is exactly what the prophet Malachi says in Malachi 4:5: "Behold I send you Elijah the prophet before the great and terrible day of the Lord comes." Amplified Bible. 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 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 accomplishing a great work that was only foreshadowed by the work of Elijah and John the Baptist. We joyfully await this glorious prospect.