Charles Ellicott Commentary Matthew 11:14

"And if ye are willing to receive [it,] this is Elijah, that is to come." — Matthew 11:14 (ASV)

This is Elijah — The words of Malachi had led people to expect the personal reappearance of the great Tishbite as the immediate forerunner of the Christ (Malachi 4:5). This was the teaching of the scribes at that time (Matthew 17:10; John 1:21), and it has lingered as a tradition in Judaism down to our own time, where a vacant chair is placed for Elijah at all great solemnities.

Even some Christian interpreters have cherished the belief that Elijah will appear in person before the Lord's second Advent. However, the true meaning of Malachi’s words had already been suggested by the angel in Luke 1:17, who said of John, He shall go before him in the spirit and power of Elias. Our Lord now distinctly confirms this understanding.

The words if you are willing to receive it imply that our Lord knew He was correcting a popular and deeply-held belief. He was essentially saying that if you can accept the truth that John was, in fact, doing the work of Elijah, then you need not look for any other in the future.