What all things did Elijah restore in Mark 9:12?
It might be suggested that John the Baptist, a figure of Elijah, did indeed "restore all things". He accomplished this in the sense that the Old Testament had ended without resolution in the Book of Malachi. John was the last of the Old Testament prophets:
Mark 9:12a, 13: "And [Christ] said to them, 'Elijah [John the Baptist] does first come and restore all things... 13But I say to you that Elijah [John] has indeed come, and they did to him whatever they wished [executed him], just as it is written of him.””
Similarly, in the Gospel of Luke:
Luke 1:16-17: “[John] 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.”
John rekindled that which remained silent for nearly 400 years at the completion of the Old Testament:
Luke 16:16: “The Law and the Prophets were proclaimed until John; since that time the gospel of the kingdom of God has been preached"
After this, it could then be brought to fulfillment by Christ's Crucifixion (death of the Testator, Heb. 8:13, 9:16), where it was nailed to the Cross (Col. 2:14). Once John began to restore faith in Israel, Christ would complete the mission bringing the Old Covenant to an end through His death.