“Remember the law of Moses My servant, even the statutes and ordinances which I commanded him in Horeb for all Israel. 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 4:4-6 (NASB)
With this calling to remember the law of Moses, and this prediction that the prophet Elijah will be sent before the coming of the Lord Himself, the prophecy of the Old Testament is brought to a close. After Malachi, no other prophet arose in Israel until the time was fulfilled when the Elijah predicted by him appeared in John the Baptist, and immediately afterwards the Lord came to His temple, that is to say, the incarnate Son of God to His own possession, to make all who receive Him children of God. . .
Law and prophets bore witness of Christ, and Christ came not to destroy the law or the prophets, but to fulfill them. Upon the Mount of Christ’s Transfiguration, therefore, there appeared both Moses, the founder of the law and mediator of the old covenant, and Elijah the prophet, as the restorer of the law in Israel, to talk with Jesus of His decrease which He was to accomplish in Jerusalem, for a practical testimony to the apostles and to us all, that Jesus Christ, who laid down His life for us, to bear our sin and redeem us from the curse of the law, was the beloved Son of the Father, whom we are to hear, that by believing in His name we may become children of God and heirs of everlasting life.
C. F. Keil, Commentary On Malachi