Commentaries:
Robertson's Word Pictures (NT)
Elijah is come already (Eleiav 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.
Other Robertson's Word Pictures (NT) entries containing Matthew 17:12:
Matthew 11:14
Mark 9:12
Luke 1:17
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