‘And he answered and said, “Elijah is indeed coming, and will restore all things, but I say to you, that Elijah is come already, and they knew him not, but did to him whatever they would. Even so will the Son of man also suffer of them.” '
In His reply Jesus first confirms that the promise that Elijah would come and ‘restore all things' was true. ‘Restore all things' is probably quoting the Scribal viewpoint, without necessarily accepting their interpretation of it (it is not found in Scriptures concerning Elijah, but Sir 48:10 paraphrases Malachi 4:6 as ‘to restore the tribes of Jacob'. Compare Isaiah 49:6 where ‘the preserved of Israel' are in mind). But then He pointed out that that had already happened. Elijah had come (compare Matthew 11:14). But the Scribes had failed to recognise him as well (because he had not restored things in their favour and exalted them and their teaching), and thus they had ‘treated him as they would'. This last is a typically Jewish description representing the self-will of evil men (compare Daniel 8:4; Daniel 11:3; Daniel 11:16).
So the Scribes had failed to recognise the very one of whom they had spoken, and they had caused him to suffer just as they will also cause the Son of Man to suffer. Indeed their very treatment of Elijah means that such treatment must be anticipated for the Son of Man as well. If they fail to recognise the one, they will not recognise the other (compare here Matthew 21:23-27).