Albert Barnes Commentary Matthew 17:12

"but I say into you, that Elijah is come already, and they knew him not, but did unto him whatsoever they would. Even so shall the Son of man also suffer of them." — Matthew 17:12 (ASV)

Elias is come. That is, John the Baptist has come in the spirit and power of Elias. See Luke 1:17.

They have done whatsoever they listed. The word list is an old English word, signifying to choose, to desire, to be inclined. See John 3:8. It means here that they had done to John as they pleased; that is, they had put him to death (Matthew 14:10).

Mark adds (Mark 9:12) that Jesus told them that it was written of the Son of man that he must suffer many things and be set at nought. This was written of him particularly in the fifty-third chapter of Isaiah. To be set at nought, is to be esteemed as worthless or as nothing; to be cast out and despised. No prophecy was ever more strikingly fulfilled. See Luke 23:11.