Elijah is come already (Eleia hdh hlqen). Thus Jesus identifies John the Baptist with the promise in Malachi, though not the real Elijah in person which John denied ( John 1:21 ). They knew him not (ouk epignwsan auton). Second aorist active indicative of epiginwskw, to recognize. Just as they do not know Jesus now ( John 1:26 ). They killed John as they will Jesus the Son of Man.