Messianic Prophecy: The forerunner will be Elijah returned:

Prophecy:

“Behold, I am going to send you Elijah the prophet before the coming of the great and terrible day of the LORD. He will restore the hearts of the fathers to their children and the hearts of the children to their fathers, so that I will not come and smite the land with a curse.”

Malachi 4:5-6

Fulfillment:

And if you are willing to accept it, John himself is Elijah who was to come.

Matthew 11:14

And He answered and said, “Elijah is coming and will restore all things; but I say to you that Elijah already came, and they did not recognize him, but did to him whatever they wished. So also the Son of Man is going to suffer at their hands.” Then the disciples understood that He had spoken to them about John the Baptist.

Matthew 17:11-13

They asked Him, saying, “Why is it that the scribes say that Elijah must come first?” And He said to them, “Elijah does first come and restore all things. And yet how is it written of the Son of Man that He will suffer many things and be treated with contempt? But I say to you that Elijah has indeed come, and they did to him whatever they wished, just as it is written of him.”

Mark 9:11-13

And he will turn many of the sons of Israel back to the Lord their God. It is he who will go as a forerunner before Him in the spirit and power of Elijah, TO TURN THE HEARTS OF THE FATHERS BACK TO THE CHILDREN, and the disobedient to the attitude of the righteous, so as to make ready a people prepared for the Lord.”

Luke 1:16-17