Albert Barnes on Matthew 17:11

Elias truly shall first come, and restore all things, He did not mean by this that Elijah was yet to come for he directly tells them that he had come; but he meant to affirm that it was a true doctrine which the scribes taught, that Elijah would appear before the coming of the Messiah. To restore, means to put into the former situation. See Mt 12:13. Hence it means, to heal, to correct, to put in proper order. Here it means that Elijah would put things in a proper state; be the instrument of reforming the people; of restoring them, in some measure, to proper notions about the Messiah, and preparing them for his coming. Before the coming of John, their views were erroneous, their expectations worldly, and their conduct exceedingly depraved. He corrected many of their notions about the Messiah, Mt 3:1 and was the instrument of an extensive reformation; and thus restored them, in some degree, to correct notions of their own economy and of the Messiah, and to a preparation for his advent.