Then they asked him, ‘Why do the scribes say that Elijah must come first?’ He said to them, ‘Elijah is indeed coming first to restore all things. How then is it written about the Son of Man, that he is to go through many sufferings and be treated with contempt? But I tell you that Elijah has come, and they did to him whatever they pleased, as it is written about him.’ (Mark 9: 11-13)

Matthew's account of this exchange is clearer to me. There Jesus explains that Elijah has already come but has not been recognized. The disciples then understood "he was speaking to them about John the Baptist." (Matthew 17: 13).

Elijah was a prophet to the Northern Kingdom of Israel roughly 900 years before. (Books of Kings) What Moses had founded, Elijah had sought to reform. By the First Century it was expected that Elijah's return would prepare the way for the Messiah.

Writing in the generation after the crucifixion of Jesus the great Rabbi Johanan ben Zakkai explained that Elijah's return would restore the purity of the people. Other Rabbis saw Elijah returning to resolve all confusion and dispute and thereby bring peace.

In Jewish apocalyptic literature Elijah performs seven dramatic miracles including raising Moses from the dead along with the entire Exodus generation. In this tradition Elijah successfully argues with God to forgive those condemned to hell.

Instead Jesus says, the prophet and precursor has already come but was not recognized. The prophecy has been fulfilled, but not as expected. Elijah has come, was widely misunderstood, and was killed by a weak ruler at the request of a dancing girl at the bidding of a vindictive adulteress.