What does Matthew 11:14 mean?

Malachi is the final book of what we now call the Old Testament. Malachi, like all the prophets, delivered the message God gave him to say. These included statements about what would happen in the distant future. Malachi lived hundreds of years before the births of John the Baptist and Jesus, but he wrote what God said about them both:

"Behold, I send my messenger, and he will prepare the way before me. And the Lord whom you seek will suddenly come to his temple; and the messenger of the covenant in whom you delight, behold, he is coming, says the LORD of hosts" (Malachi 3:1).

"Behold, I will send you Elijah the prophet before the great and awesome day of the LORD comes" (Malachi 4:5).

Luke's gospel quotes the angel of the Lord describing the work John the Baptist will do to his father. This uses the same phrasing found in Malachi: "And he will turn many of the children of Israel to the Lord their God, and he will go before him in the spirit and power of Elijah, to turn the hearts of the fathers to the children, and the disobedient to the wisdom of the just, to make ready for the Lord a people prepared" (Luke 1:16–17).

John the Baptist himself denied that he was literally Elijah (John 1:21); he refuted any suggestion that he was that prophet reborn or returned to earth in some way. Jesus, though, insists that John was the fulfillment of Malachi's prophecy that Elijah would be sent to prepare the way for Christ. John certainly came "in the spirit and power of Elijah," as the angel said.

Jesus adds "if you are willing to accept it," implying that He knew not everyone would grasp that this was true or what it meant. If John the Baptist was the fulfillment of the promise of the return of Elijah to usher in the day of the Lord, it means Jesus was claiming to be the Lord.