John Gill Commentary Mark 9:12

"And he said unto them, Elijah indeed cometh first, and restoreth all things: and how is it written of the Son of man, that he should suffer many things and be set at nought?" — Mark 9:12 (ASV)

And he answered, and told them
Allowing that their observation was right, and that this was the sense of the Scribes, and that there was something of truth in it, when rightly understood:

Elias verily comes first, and restores all things :
(See Gill on Matthew 17:11);

and how it is written of the son of man, that he must suffer many
things, and be set at nought .
The sense of Christ is, that John the Baptist, whom he means by Elias, comes first, and restores all things: and among the rest of the things he sets right, this is one, and not of the least; namely, that he gives the true sense of such passages of the sacred writings, which related to the contemptuous usage, rejection, and sufferings of the Messiah; as that in these he was the Lamb of God typified in the sacrifices of the law, who by his sufferings and death takes away the sin, of the world; and therefore he exhorted and directed those to whom he ministered, to look unto him, and believe in him; see (John 1:29) (Acts 19:4) .