Elias is come already. The mystical, not the real, Elias, even John the Baptist, who came in the spirit and power of Elias (Luke 1:17). Christ is here speaking of the past, as in the preceding verse he spake of the future.
The common Jewish interpretation confused the two events and the two personages, reducing them to one. And this mistake has been committed by many modern expositors. They knew him not. They did not recognize his true character and the import of his mission.
Though they gathered round him and listened to his preaching and denunciations, very few saw in him the precursor of the Messiah, and many, misunderstanding his austere, self-denying life, deemed him to be possessed by a devil (Matthew 11:18).
They have done unto him. John suffered a long imprisonment, and was eventually murdered; and though Herod was primarily answerable for these doings, the people were virtually guilty, in that they consented to the injurious treatment and made no effort in his favour.
Likewise … also. Taking occasion from the mention of the Baptist's fate, Jesus foretells his own sufferings and death, endeavouring to make the apostles familiar with the idea of a dying as well as a conquering Messiah.