Albert Barnes on Matthew 17:12

Elias is come. That is, John the Baptist has come, in the spirit and power of Elias. See Lk 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 Jn 3:8. It means here, that they had done to John as they pleased; i.e., they had put him to death, Mt 14:10.

Mark adds, Mk 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 Lk 23:11.

(o) "suffer of them" Mt 16:21